PL/SQL stands for Procedural Language extension of SQL. It is a combination of SQL along with the procedural features of programming languages and it enhances the capabilities of SQL by injecting the procedural functionality, like conditional or looping statements, into the set-oriented SQL structure.
Advantages of PL/SQL
- Procedural Language Capability: PL/SQL consists of procedural language constructs such as conditional statements (if else statements) and loops like (FOR loops).
- Block Structures: PL/SQL consists of blocks of code, which can be nested within each other. Each block forms a unit of a task or a logical module. PL/SQL Blocks can be stored in the database and reused.
- Better Performance: PL/SQL engine processes multiple SQL statements simultaneously as a single block, thereby reducing network traffic.
- Exception Handling: PL/SQL handles exceptions (or errors) effectively during the execution of a PL/SQL program. Once an exception is caught, specific actions can be taken depending upon the type of the exception or it can be displayed to the user with a message.
PL/SQL can only use SELECT, DML(INSERT, UPDATE, DELETE) and TC(COMMIT, ROLLBACK, SAVEPOINT) statements, DDL (CREATE, ALTER, DROP) and DCL(GRANT, REVOKE) cannot be used directly. Any DDL/DCL however, can be executed from PL/SQL when embedded in an EXECUTE IMMEDIATE statement.
Each PL/SQL program consists of SQL and PL/SQL statements which form a PL/SQL block. A PL/SQL Block consists of three sections:
Declaration Section: This section is optional and it starts with the reserved keyword DECLARE. This section is used to declare any placeholders like variables, constants, records and cursors, which are used to manipulate data in the execution section.
Execution Section: This section is mandatory and it starts with the reserved keyword BEGIN and ends with END. This section is where the program logic is written to perform any task. The programmatic constructs like loops, conditional statement and SQL statements form the part of execution section.
Exception Section: The section is optional and it starts with the reserved keyword EXCEPTION. Any exception in the program can be handled in this section, so that the PL/SQL Blocks terminates gracefully. If the PL/SQL Block contains exceptions that cannot be handled, the block terminates abruptly with errors.
Every statement in the above three sections must end with a ; (semicolon). PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.
This is how PL/SQL looks.
Placeholders are temporary storage area. Placeholders can be any of Variables, Constants and Records. Oracle defines placeholders to store data temporarily, which are used to manipulate data during the execution of a PL SQL block.
Depending on the kind of data you want to store, you can define placeholders with a name and a datatype. Few of the datatypes used to define placeholders are as given below.
The placeholders, that store the values, can change through the PL/SQL Block.
The General Syntax to declare a variable is:
- variable_name is the name of the variable.
- datatype is a valid PL/SQL datatype.
- NOT NULL is an optional specification on the variable. If NOT NULL is specified, you must provide the initial value.
- value or DEFAULT value is also an optional specification, where you can initialize a variable.
- Each variable declaration is a separate statement and must be terminated by a semicolon.
The below example declares two variables, one of which is a not null.
The value of a variable can change in the execution or exception section of the PL/SQL Block. We can assign values to variables in the two ways given below.
1) Directly assign value to variable.
2) Assign values to variables directly from the database columns.
The example below will get the salary of an employee with id '12345' and display it on the screen.
NOTE: The slash '/' indicates to execute the above PL/SQL Block.
Records are composite datatypes, which contains a combination of different scalar datatypes like char, varchar, number etc. Each scalar data types in the record holds a value. A record can store values of a row in a table.
A datatype can be declared in the same way as you create a table, like col_name_1. If a field is based on a column from database table, you can define the datatype as col_name_2. You can also use %type method to declare datatype of variable and constant. Similar to %type, if all the fields of a record are based on the columns of a table, it can be declared by using %rowtype method.
Declaring the record as a ROWTYPE Advantages: 1) Do not need to explicitly declare variables for all the columns in a table. 2) If the column specification in the database table is altered, the code does not need to update.
Disadvantage: 1) When a record is created as a ROWTYPE, fields will be created for all the columns in the table and memory will be used to create the datatype for all the fields.
Assign values to record Similar to variable, you can assign value to record either by direct assign or through the SELECT statements
If the records is declared as ROWTYPE, SELECT * can be used to assign values.
The column value of the record can be retrieved as below syntax
Scope of Variables and Records
PL/SQL allows the nesting of Blocks within Blocks i.e, the Execution section of an outer block can contain inner blocks. Variables which are accessible to an outer Block are also accessible to all nested inner blocks; however, the variables declared in the inner blocks are not accessible to the outer blocks.
Based on their declaration we can classify variables into two types.
- Local variables - These are declared in a inner block and cannot be referenced by outer blocks.
- Global variables - These are declared in an outer block and can be referenced by its itself and by its inner blocks.
In the below example, two variables are created in the outer block and assigning their product to the third variable created in the inner block. The variables 'var_num1' and 'var_num2' can be accessed anywhere in the block; however, the variable 'var_result' is declared in the inner block, so it cannot be accessed in the outer block.
As the name implies a constant is a value used in a PL/SQL Block that remains unchanged throughout the program. A constant is a user-defined literal value. You can declare a constant and use it instead of actual value.
You must assign a value to the constant while declaring it. If you assign a value to the constant later, Oracle will prompt exception.
PL/SQL Conditional Statements
PL/SQL supports programming language features like conditional statements, iterative statements.
The syntax of conditional statements:
Note: be aware of the keyword ELSIF, there is no 'E' before 'IF'.
PL/SQL Iterative Statements
An iterative statements are used when you want to repeat the execution of one or more statements for specified number of times. There are three types of loops in PL/SQL:
1. Simple Loop A Simple Loop is used when a set of statements is to be executed at least once before the loop terminates. An EXIT condition must be specified in the loop, otherwise the loop will get into an infinite number of iterations. When the EXIT condition is satisfied the process exits from the loop.
Note: a) Initialize a variable before the loop body. b) Increment the variable in the loop. c) Use a EXIT WHEN statement to exit from the Loop. If you use a EXIT statement without WHEN condition, the statements in the loop is executed only once.
2. While Loop A WHILE LOOP is used when a set of statements has to be executed as long as a condition is true. The condition is evaluated at the beginning of each iteration. The iteration continues until the condition becomes false.
Note: a) Initialize a variable before the loop body. b) Increment the variable in the loop. c) EXIT WHEN statement and EXIT statements can be used in while loops it is seldom used.
3. FOR Loop A FOR LOOP is used to execute a set of statements for a pre-determined number of times. Iteration occurs between the start and end integer values given. The counter is always incremented by 1. The loop exits when the counter reaches the value of the end integer.
Note: a) The counter variable is implicitly declared in the declaration section, so it's not necessary to declare it explicitly. b) The counter variable is incremented by 1 and does not need to be incremented explicitly. c) EXIT WHEN statement and EXIT statements can be used in FOR loops but it is seldom used.
A cursor contains information on a select statement and the rows of data accessed by it. This temporary work area is used to store the data retrieved from the database, and manipulate this data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the cursor holds is called the active set.
There are two types of cursors in PL/SQL:
When you execute DML statements like DELETE, INSERT, UPDATE and SELECT..INTO statements, implicit statements are created to process these statements.
Oracle provides few attributes called as implicit cursor attributes to check the status of DML operations. The cursor attributes available are %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN.
For example, When you execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us whether any rows are affected and how many have been affected. When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.
The status of the cursor for each of these attributes are defined in the below table.
|SQL%FOUND||The return value is TRUE, if the DML statements like INSERT, DELETE and UPDATE affect at least one row and if SELECT ….INTO statement return at least one row. |
The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE do not affect row and if SELECT….INTO statement do not return a row.
|SQL%NOTFOUND||The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE at least one row and if SELECT ….INTO statement return at least one row. |
The return value is TRUE, if a DML statement like INSERT, DELETE and UPDATE do not affect even one row and if SELECT ….INTO statement does not return a row.
|SQL%ROWCOUNT||Return the number of rows affected by the DML operations INSERT, DELETE, UPDATE, SELECT|
|SQL%ISOPEN||Always return FALSE|
uses implicit cursor attributes:
In the above PL/SQL Block, the salaries of all the employees in the 'employee' table are updated. If none of the employee's salary are updated we get a message 'None of the salaries where updated'. Else we get a message like for example, 'Salaries for 100 employees are updated' if there are 100 rows in 'employee' table.
An explicit cursor is defined in the declaration section of the PL/SQL Block. It must be created when you are executing a SELECT statement that returns more than one row. Even though the cursor stores multiple records, only one record can be processed at a time, which is called as current row. When you fetch a row the current row position moves to next row.
There are four steps in using an Explicit Cursor.
- DECLARE the cursor in the declaration section.
- OPEN the cursor in the Execution Section.
- FETCH the data from cursor into PL/SQL variables or records in the Execution Section.
- CLOSE the cursor in the Execution Section before you end the PL/SQL Block.
Using Cursor When a cursor is opened, the first row becomes the current row. When the data is fetched it is copied to the record or variables and the logical pointer moves to the next row and it becomes the current row. On every fetch statement, the pointer moves to the next row. If you want to fetch after the last row, the program will throw an error. When there is more than one row in a cursor we can use loops along with explicit cursor attributes to fetch all the records.
- We can fetch the rows in a cursor to a PL/SQL Record or a list of variables created in the PL/SQL Block.
- If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor.
- If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor.
- When we try to open a cursor which is not closed in the previous operation, it throws exception.
- When we try to fetch a cursor after the last operation, it throws exception.
Oracle provides some attributes known as Explicit Cursor Attributes to control the data processing while using cursors. We use these attributes to avoid errors while accessing cursors through OPEN, FETCH and CLOSE Statements.
|Cursor_name%FOUND||TRUE, if fetch statement returns at least one row. |
FALSE, if fetch statement doesn’t return a row.
|Cursor_name%NOTFOUND||TRUE, , if fetch statement doesn’t return a row. |
FALSE, if fetch statement returns at least one row.
|Cursor_name%ROWCOUNT||The number of rows fetched by the fetch statement. |
If no row is returned, the PL/SQL statement returns an error.
|Cursor_name%ISOPEN||TRUE, if the cursor is already open in the program. |
FALSE, if the cursor is not opened in the program.
Cursor with a Simple Loop:
The cursor attribute %ISOPEN is used to check if the cursor is open, if the condition is true the program does not open the cursor again. The cursor attribute %NOTFOUND is used to check whether the fetch returned any row. If there is no row found, the program would exit. Typically, when the cursor reach the last row, no more row can be fetched.
Cursor with a While Loop:
Using %FOUND to evaluate if the first fetch statement returned a row, if TRUE, the program moves into the while loop. Inside the loop, use fetch statement again to process the next row. If the fetch statement is not executed once before the while loop, the while condition will return false in the first instance and the while loop is skipped.
Cursor with a FOR Loop: When using FOR LOOP, you do not need to declare a record or variables to store the cursor values, do not need to open, fetch and close the cursor. These functions are accomplished by the FOR LOOP automatically.
When the FOR loop is processed a record 'rec_emp' of structure 'cur_emp' gets created, the cursor is opened, the rows are fetched to the record 'rec_emp' and the cursor is closed after the last row is processed. By using FOR Loop, you can reduce the number of lines in the program.
PL/SQL Exception Handling
When calling store procedure from the PL/SQL Block, you simply use the store procedure name to call. If you prefix the 'EXECUTE' keyword in front of the store procedure name, you will receive an error.
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Overview of PL/SQL
The limits of my language mean the limits of my world. --Ludwig Wittgenstein
This chapter surveys the main features of PL/SQL and points out the advantages they offer. It also acquaints you with the basic concepts behind PL/SQL and the general appearance of PL/SQL programs. You see how PL/SQL bridges the gap between database technology and procedural programming languages.
This chapter discusses the following topics:
- Understanding the Main Features of PL/SQL
- PL/SQL Architecture
- Advantages of PL/SQL
Understanding the Main Features of PL/SQL
A good way to get acquainted with PL/SQL is to look at a sample program. The program below processes an order for a tennis racket. First, it declares a variable of type to store the quantity of tennis rackets on hand. Then, it retrieves the quantity on hand from a database table named . If the quantity is greater than zero, the program updates the table and inserts a purchase record into another table named . Otherwise, the program inserts an out-of-stock record into the table.-- available online in file 'examp1' DECLARE qty_on_hand NUMBER(5); BEGIN SELECT quantity INTO qty_on_hand FROM inventory WHERE product = 'TENNIS RACKET' FOR UPDATE OF quantity; IF qty_on_hand > 0 THEN -- check quantity UPDATE inventory SET quantity = quantity - 1 WHERE product = 'TENNIS RACKET'; INSERT INTO purchase_record VALUES ('Tennis racket purchased', SYSDATE); ELSE INSERT INTO purchase_record VALUES ('Out of tennis rackets', SYSDATE); END IF; COMMIT; END;
With PL/SQL, you can use SQL statements to manipulate Oracle data and flow-of-control statements to process the data. You can also declare constants and variables, define procedures and functions, and trap runtime errors. Thus, PL/SQL combines the data manipulating power of SQL with the data processing power of procedural languages.
PL/SQL is a block-structured language. That is, the basic units (procedures, functions, and anonymous blocks) that make up a PL/SQL program are logical blocks, which can contain any number of nested sub-blocks. Typically, each logical block corresponds to a problem or subproblem to be solved. Thus, PL/SQL supports the divide-and-conquer approach to problem solving called stepwise refinement.
A block (or sub-block) lets you group logically related declarations and statements. That way, you can place declarations close to where they are used. The declarations are local to the block and cease to exist when the block completes.
As Figure 1-1 shows, a PL/SQL block has three parts: a declarative part, an executable part, and an exception-handling part. (In PL/SQL, a warning or error condition is called an exception.) Only the executable part is required.
The order of the parts is logical. First comes the declarative part, in which items can be declared. Once declared, items can be manipulated in the executable part. Exceptions raised during execution can be dealt with in the exception-handling part.
Figure 1-1 Block Structure
Text description of the illustration pls81001_block_structure.gif
You can nest sub-blocks in the executable and exception-handling parts of a PL/SQL block or subprogram but not in the declarative part. Also, you can define local subprograms in the declarative part of any block. However, you can call local subprograms only from the block in which they are defined.
Variables and Constants
PL/SQL lets you declare constants and variables, then use them in SQL and procedural statements anywhere an expression can be used. However, forward references are not allowed. So, you must declare a constant or variable before referencing it in other statements, including other declarative statements.
Variables can have any SQL datatype, such as , , or , or any PL/SQL datatype, such as or . For example, assume that you want to declare a variable named to hold 4-digit numbers and a variable named to hold the Boolean value or . You declare these variables as follows:part_no NUMBER(4); in_stock BOOLEAN;
You can also declare nested tables, variable-size arrays (varrays for short), and records using the , , and composite datatypes.
Assigning Values to a Variable
You can assign values to a variable in three ways. The first way uses the assignment operator (), a colon followed by an equal sign. You place the variable to the left of the operator and an expression (which can include function calls) to the right. A few examples follow:tax := price * tax_rate; valid_id := FALSE; bonus := current_salary * 0.10; wages := gross_pay(emp_id, st_hrs, ot_hrs) - deductions;
The second way to assign values to a variable is by selecting (or fetching) database values into it. In the example below, you have Oracle compute a 10% bonus when you select the salary of an employee. Now, you can use the variable in another computation or insert its value into a database table.SELECT sal * 0.10 INTO bonus FROM emp WHERE empno = emp_id;
The third way to assign values to a variable is by passing it as an or parameter to a subprogram. As the following example shows, an parameter lets you pass initial values to the subprogram being called and return updated values to the caller:DECLARE my_sal REAL(7,2); PROCEDURE adjust_salary (emp_id INT, salary IN OUT REAL) IS ... BEGIN SELECT AVG(sal) INTO my_sal FROM emp; adjust_salary(7788, my_sal); -- assigns a new value to my_sal
Declaring a constant is like declaring a variable except that you must add the keyword and immediately assign a value to the constant. Thereafter, no more assignments to the constant are allowed. In the following example, you declare a constant named :credit_limit CONSTANT REAL := 5000.00;
Oracle uses work areas to execute SQL statements and store processing information. A PL/SQL construct called a cursor lets you name a work area and access its stored information. There are two kinds of cursors: implicit and explicit. PL/SQL implicitly declares a cursor for all SQL data manipulation statements, including queries that return only one row. For queries that return more than one row, you can explicitly declare a cursor to process the rows individually. An example follows:DECLARE CURSOR c1 IS SELECT empno, ename, job FROM emp WHERE deptno = 20;
The set of rows returned by a multi-row query is called the result set. Its size is the number of rows that meet your search criteria. As Figure 1-2 shows, an explicit cursor "points" to the current row in the result set. This allows your program to process the rows one at a time.
Figure 1-2 Query Processing
Text description of the illustration pls81003_query_processing.gif
Multi-row query processing is somewhat like file processing. For example, a COBOL program opens a file, processes records, then closes the file. Likewise, a PL/SQL program opens a cursor, processes rows returned by a query, then closes the cursor. Just as a file pointer marks the current position in an open file, a cursor marks the current position in a result set.
You use the , , and statements to control a cursor. The statement executes the query associated with the cursor, identifies the result set, and positions the cursor before the first row. The statement retrieves the current row and advances the cursor to the next row. When the last row has been processed, the statement disables the cursor.
Cursor FOR Loops
In most situations that require an explicit cursor, you can simplify coding by using a cursor loop instead of the , , and statements. A cursor loop implicitly declares its loop index as a record that represents a row fetched from the database. Next, it opens a cursor, repeatedly fetches rows of values from the result set into fields in the record, then closes the cursor when all rows have been processed. In the following example, the cursor loop implicitly declares as a record:DECLARE CURSOR c1 IS SELECT ename, sal, hiredate, deptno FROM emp; ... BEGIN FOR emp_rec IN c1 LOOP ... salary_total := salary_total + emp_rec.sal; END LOOP;
To reference individual fields in the record, you use dot notation, in which a dot () serves as the component selector.
Like a cursor, a cursor variable points to the current row in the result set of a multi-row query. But, unlike a cursor, a cursor variable can be opened for any type-compatible query. It is not tied to a specific query. Cursor variables are true PL/SQL variables, to which you can assign new values and which you can pass to subprograms stored in an Oracle database. This gives you more flexibility and a convenient way to centralize data retrieval.
Typically, you open a cursor variable by passing it to a stored procedure that declares a cursor variable as one of its formal parameters. The following procedure opens the cursor variable for the chosen query:PROCEDURE open_cv (generic_cv IN OUT GenericCurTyp,choice NUMBER) IS BEGIN IF choice = 1 THEN OPEN generic_cv FOR SELECT * FROM emp; ELSIF choice = 2 THEN OPEN generic_cv FOR SELECT * FROM dept; ELSIF choice = 3 THEN OPEN generic_cv FOR SELECT * FROM salgrade; END IF; ... END;
PL/SQL variables and cursors have attributes, which are properties that let you reference the datatype and structure of an item without repeating its definition. Database columns and tables have similar attributes, which you can use to ease maintenance. A percent sign () serves as the attribute indicator.
The attribute provides the datatype of a variable or database column. This is particularly useful when declaring variables that will hold database values. For example, assume there is a column named in a table named . To declare a variable named that has the same datatype as column , use dot notation and the attribute, as follows:my_title books.title%TYPE;
Declaring with has two advantages. First, you need not know the exact datatype of . Second, if you change the database definition of (make it a longer character string for example), the datatype of changes accordingly at run time.
In PL/SQL, records are used to group data. A record consists of a number of related fields in which data values can be stored. The attribute provides a record type that represents a row in a table. The record can store an entire row of data selected from the table or fetched from a cursor or cursor variable.
Columns in a row and corresponding fields in a record have the same names and datatypes. In the example below, you declare a record named . Its fields have the same names and datatypes as the columns in the table.DECLARE dept_rec dept%ROWTYPE; -- declare record variable
You use dot notation to reference fields, as the following example shows:my_deptno := dept_rec.deptno;
If you declare a cursor that retrieves the last name, salary, hire date, and job title of an employee, you can use to declare a record that stores the same information, as follows:DECLARE CURSOR c1 IS SELECT ename, sal, hiredate, job FROM emp; emp_rec c1%ROWTYPE; -- declare record variable that represents -- a row fetched from the emp table
When you execute the statementFETCH c1 INTO emp_rec;
the value in the column of the table is assigned to the field of , the value in the column is assigned to the field, and so on. Figure 1-3 shows how the result might appear.
Figure 1-3 %ROWTYPE Record
Text description of the illustration pls81002_rowtype_record.gif
Control structures are the most important PL/SQL extension to SQL. Not only does PL/SQL let you manipulate Oracle data, it lets you process the data using conditional, iterative, and sequential flow-of-control statements such as , , , , , and . Collectively, these statements can handle any situation.
Often, it is necessary to take alternative actions depending on circumstances. The statement lets you execute a sequence of statements conditionally. The clause checks a condition; the clause defines what to do if the condition is true; the clause defines what to do if the condition is false or null.
Consider the program below, which processes a bank transaction. Before allowing you to withdraw $500 from account 3, it makes sure the account has sufficient funds to cover the withdrawal. If the funds are available, the program debits the account. Otherwise, the program inserts a record into an audit table.-- available online in file 'examp2' DECLARE acct_balance NUMBER(11,2); acct CONSTANT NUMBER(4) := 3; debit_amt CONSTANT NUMBER(5,2) := 500.00; BEGIN SELECT bal INTO acct_balance FROM accounts WHERE account_id = acct FOR UPDATE OF bal; IF acct_balance >= debit_amt THEN UPDATE accounts SET bal = bal - debit_amt WHERE account_id = acct; ELSE INSERT INTO temp VALUES (acct, acct_balance, 'Insufficient funds'); -- insert account, current balance, and message END IF; COMMIT; END;
To choose among several values or courses of action, you can use constructs. The CASE expression evaluates a condition and returns a value for each case. The case statement evaluates a condition and performs an action (which might be an entire PL/SQL block) for each case.-- This CASE statement performs different actions based -- on a set of conditional tests. CASE WHEN shape = 'square' THEN area := side * side; WHEN shape = 'circle' THEN BEGIN area := pi * (radius * radius); DBMS_OUTPUT.PUT_LINE('Value is not exact because pi is irrational.'); END; WHEN shape = 'rectangle' THEN area := length * width; ELSE BEGIN DBMS_OUTPUT.PUT_LINE('No formula to calculate area of a' || shape); RAISE PROGRAM_ERROR; END; END CASE;
A sequence of statements that uses query results to select alternative actions is common in database applications. Another common sequence inserts or deletes a row only if an associated entry is found in another table. You can bundle these common sequences into a PL/SQL block using conditional logic.
statements let you execute a sequence of statements multiple times. You place the keyword before the first statement in the sequence and the keywords after the last statement in the sequence. The following example shows the simplest kind of loop, which repeats a sequence of statements continually:LOOP -- sequence of statements END LOOP;
The statement lets you specify a range of integers, then execute a sequence of statements once for each integer in the range. For example, the following loop inserts 500 numbers and their square roots into a database table:FOR num IN 1..500 LOOP INSERT INTO roots VALUES (num, SQRT(num)); END LOOP;
The statement associates a condition with a sequence of statements. Before each iteration of the loop, the condition is evaluated. If the condition is true, the sequence of statements is executed, then control resumes at the top of the loop. If the condition is false or null, the loop is bypassed and control passes to the next statement.
In the following example, you find the first employee who has a salary over $2500 and is higher in the chain of command than employee 7499:-- available online in file 'examp3' DECLARE salary emp.sal%TYPE := 0; mgr_num emp.mgr%TYPE; last_name emp.ename%TYPE; starting_empno emp.empno%TYPE := 7499; BEGIN SELECT mgr INTO mgr_num FROM emp WHERE empno = starting_empno; WHILE salary <= 2500 LOOP SELECT sal, mgr, ename INTO salary, mgr_num, last_name FROM emp WHERE empno = mgr_num; END LOOP; INSERT INTO temp VALUES (NULL, salary, last_name); COMMIT; EXCEPTION WHEN NO_DATA_FOUND THEN INSERT INTO temp VALUES (NULL, NULL, 'Not found'); COMMIT; END;
The statement lets you complete a loop if further processing is impossible or undesirable. When the statement is encountered, the condition in the clause is evaluated. If the condition is true, the loop completes and control passes to the next statement. In the following example, the loop completes when the value of exceeds 25,000:LOOP ... total := total + salary; EXIT WHEN total > 25000; -- exit loop if condition is true END LOOP; -- control resumes here
The statement lets you branch to a label unconditionally. The label, an undeclared identifier enclosed by double angle brackets, must precede an executable statement or a PL/SQL block. When executed, the statement transfers control to the labeled statement or block, as the following example shows:IF rating > 90 THEN GOTO calc_raise; -- branch to label END IF; ... <<calc_raise>> IF job_title = 'SALESMAN' THEN -- control resumes here amount := commission * 0.25; ELSE amount := salary * 0.10; END IF;
Modularity lets you break an application down into manageable, well-defined modules. Through successive refinement, you can reduce a complex problem to a set of simple problems that have easy-to-implement solutions. PL/SQL meets this need with program units, which include blocks, subprograms, and packages.
PL/SQL has two types of subprograms called procedures and functions, which can take parameters and be invoked (called). As the following example shows, a subprogram is like a miniature program, beginning with a header followed by an optional declarative part, an executable part, and an optional exception-handling part:PROCEDURE award_bonus (emp_id NUMBER) IS bonus REAL; comm_missing EXCEPTION; BEGIN -- executable part starts here SELECT comm * 0.15 INTO bonus FROM emp WHERE empno = emp_id; IF bonus IS NULL THEN RAISE comm_missing; ELSE UPDATE payroll SET pay = pay + bonus WHERE empno = emp_id; END IF; EXCEPTION -- exception-handling part starts here WHEN comm_missing THEN ... END award_bonus;
When called, this procedure accepts an employee number. It uses the number to select the employee's commission from a database table and, at the same time, compute a 15% bonus. Then, it checks the bonus amount. If the bonus is null, an exception is raised; otherwise, the employee's payroll record is updated.
PL/SQL lets you bundle logically related types, variables, cursors, and subprograms into a package. Each package is easy to understand and the interfaces between packages are simple, clear, and well defined. This aids application development.
Packages usually have two parts: a specification and a body. The specification is the interface to your applications; it declares the types, constants, variables, exceptions, cursors, and subprograms available for use. The body defines cursors and subprograms and so implements the specification.
In the following example, you package two employment procedures:CREATE PACKAGE emp_actions AS -- package specification PROCEDURE hire_employee (empno NUMBER, ename CHAR, ...); PROCEDURE fire_employee (emp_id NUMBER); END emp_actions; CREATE PACKAGE BODY emp_actions AS -- package body PROCEDURE hire_employee (empno NUMBER, ename CHAR, ...) IS BEGIN INSERT INTO emp VALUES (empno, ename, ...); END hire_employee; PROCEDURE fire_employee (emp_id NUMBER) IS BEGIN DELETE FROM emp WHERE empno = emp_id; END fire_employee; END emp_actions;
Only the declarations in the package specification are visible and accessible to applications. Implementation details in the package body are hidden and inaccessible.
Packages can be compiled and stored in an Oracle database, where their contents can be shared by many applications. When you call a packaged subprogram for the first time, the whole package is loaded into memory. So, subsequent calls to related subprograms in the package require no disk I/O. Thus, packages can enhance productivity and improve performance.
Data abstraction lets you extract the essential properties of data while ignoring unnecessary details. Once you design a data structure, you can forget the details and focus on designing algorithms that manipulate the data structure.
The collection types and allow you to declare index-by tables, nested tables, and variable-size arrays (varrays for short). A collection is an ordered group of elements, all of the same type. Each element has a unique subscript that determines its position in the collection.
To reference an element, use standard subscripting syntax. For example, the following call references the fifth element in the nested table (of type ) returned by function :DECLARE TYPE Staff IS TABLE OF Employee; staffer Employee; FUNCTION new_hires (hiredate DATE) RETURN Staff IS BEGIN ... END; BEGIN staffer := new_hires('10-NOV-98')(5); ... END;
Collections work like the arrays found in most third-generation programming languages. Also, collections can be passed as parameters. So, you can use them to move columns of data into and out of database tables or between client-side applications and stored subprograms.
You can use the attribute to declare a record that represents a row in a table or a row fetched from a cursor. But, with a user-defined record, you can declare fields of your own.
Records contain uniquely named fields, which can have different datatypes. Suppose you have various data about an employee such as name, salary, and hire date. These items are dissimilar in type but logically related. A record containing a field for each item lets you treat the data as a logical unit.
Consider the following example:DECLARE TYPE TimeRec IS RECORD (hours SMALLINT, minutes SMALLINT); TYPE MeetingTyp IS RECORD ( date_held DATE, duration TimeRec, -- nested record location VARCHAR2(20), purpose VARCHAR2(50));
Notice that you can nest records. That is, a record can be a component of another record.
In PL/SQL, object-oriented programming is based on object types. An object type encapsulates a data structure along with the functions and procedures needed to manipulate the data. The variables that form the data structure are called attributes. The functions and procedures that characterize the behavior of the object type are called methods.
Object types reduce complexity by breaking down a large system into logical entities. This lets you create software components that are modular, maintainable, and reusable.
When you define an object type using the statement (in SQL*Plus for example), you create an abstract template for some real-world object. As the following example of a bank account shows, the template specifies only those attributes and behaviors the object will need in the application environment:CREATE TYPE Bank_Account AS OBJECT ( acct_number INTEGER(5), balance REAL, status VARCHAR2(10), MEMBER PROCEDURE open (amount IN REAL), MEMBER PROCEDURE verify_acct (num IN INTEGER), MEMBER PROCEDURE close (num IN INTEGER, amount OUT REAL), MEMBER PROCEDURE deposit (num IN INTEGER, amount IN REAL), MEMBER PROCEDURE withdraw (num IN INTEGER, amount IN REAL), MEMBER FUNCTION curr_bal (num IN INTEGER) RETURN REAL );
At run time, when the data structure is filled with values, you have created an instance of an abstract bank account. You can create as many instances (called objects) as you need. Each object has the number, balance, and status of an actual bank account.
With information hiding, you see only the details that are relevant at a given level of algorithm and data structure design. Information hiding keeps high-level design decisions separate from low-level design details, which are more likely to change.
You implement information hiding for algorithms through top-down design. Once you define the purpose and interface specifications of a low-level procedure, you can ignore the implementation details. They are hidden at higher levels. For example, the implementation of a procedure named is hidden. All you need to know is that the procedure will increase a specific employee's salary by a given amount. Any changes to the definition of are transparent to calling applications.
You implement information hiding for data structures though data encapsulation. By developing a set of utility subprograms for a data structure, you insulate it from users and other developers. That way, other developers know how to use the subprograms that operate on the data structure but not how the structure is represented.
With PL/SQL packages, you can specify whether subprograms are public or private. Thus, packages enforce data encapsulation by letting you put subprogram definitions in a black box. A private definition is hidden and inaccessible. Only the package, not your application, is affected if the definition changes. This simplifies maintenance and enhancement.
PL/SQL makes it easy to detect and process predefined and user-defined error conditions called exceptions. When an error occurs, an exception is raised. That is, normal execution stops and control transfers to the exception-handling part of your PL/SQL block or subprogram. To handle raised exceptions, you write separate routines called exception handlers.
Predefined exceptions are raised implicitly by the runtime system. For example, if you try to divide a number by zero, PL/SQL raises the predefined exception automatically. You must raise user-defined exceptions explicitly with the statement.
You can define exceptions of your own in the declarative part of any PL/SQL block or subprogram. In the executable part, you check for the condition that needs special attention. If you find that the condition exists, you execute a statement. In the example below, you compute the bonus earned by a salesperson. The bonus is based on salary and commission. So, if the commission is null, you raise the exception .DECLARE ... comm_missing EXCEPTION; -- declare exception BEGIN ... IF commission IS NULL THEN RAISE comm_missing; -- raise exception END IF; bonus := (salary * 0.10) + (commission * 0.15); EXCEPTION WHEN comm_missing THEN ... -- process the exception
The PL/SQL compilation and run-time system is a technology, not an independent product. Think of this technology as an engine that compiles and executes PL/SQL blocks and subprograms. The engine can be installed in an Oracle server or in an application development tool such as Oracle Forms or Oracle Reports. So, PL/SQL can reside in two environments:
- The Oracle database server
- Oracle tools
These two environments are independent. PL/SQL is bundled with the Oracle server but might be unavailable in some tools. In either environment, the PL/SQL engine accepts as input any valid PL/SQL block or subprogram. Figure 1-4 shows the PL/SQL engine processing an anonymous block. The engine executes procedural statements but sends SQL statements to the SQL Statement Executor in the Oracle server.
Figure 1-4 PL/SQL Engine
Text description of the illustration pls81004_plsql_engine.gif
In the Oracle Database Server
Application development tools that lack a local PL/SQL engine must rely on Oracle to process PL/SQL blocks and subprograms. When it contains the PL/SQL engine, an Oracle server can process PL/SQL blocks and subprograms as well as single SQL statements. The Oracle server passes the blocks and subprograms to its local PL/SQL engine.
Anonymous PL/SQL blocks can be embedded in an Oracle Precompiler or OCI program. At run time, the program, lacking a local PL/SQL engine, sends these blocks to the Oracle server, where they are compiled and executed. Likewise, interactive tools such as SQL*Plus and Enterprise Manager, lacking a local PL/SQL engine, must send anonymous blocks to Oracle.
Subprograms can be compiled separately and stored permanently in an Oracle database, ready to be executed. A subprogram explicitly d using an Oracle tool is called a stored subprogram. Once compiled and stored in the data dictionary, it is a schema object, which can be referenced by any number of applications connected to that database.
Stored subprograms defined within a package are called packaged subprograms. Those defined independently are called standalone subprograms. Those defined within another subprogram or within a PL/SQL block are called local subprograms, which cannot be referenced by other applications and exist only for the convenience of the enclosing block.
Stored subprograms offer higher productivity, better performance, memory savings, application integrity, and tighter security. For example, by designing applications around a library of stored procedures and functions, you can avoid redundant coding and increase your productivity.
You can call stored subprograms from a database trigger, another stored subprogram, an Oracle Precompiler application, an OCI application, or interactively from SQL*Plus or Enterprise Manager. For example, you might call the standalone procedure from SQL*Plus as follows:SQL> CALL create_dept('FINANCE', 'NEW YORK');
Subprograms are stored in parsed, compiled form. So, when called, they are loaded and passed to the PL/SQL engine immediately. Also, they take advantage of shared memory. So, only one copy of a subprogram need be loaded into memory for execution by multiple users.
A database trigger is a stored subprogram associated with a database table, view, or event. For instance, you can have Oracle fire a trigger automatically before or after an , , or statement affects a table. One of the many uses for database triggers is to audit data modifications. For example, the following table-level trigger fires whenever salaries in the table are updated:CREATE TRIGGER audit_sal AFTER UPDATE OF sal ON emp FOR EACH ROW BEGIN INSERT INTO emp_audit VALUES ... END;
The executable part of a trigger can contain procedural statements as well as SQL data manipulation statements. Besides table-level triggers, there are instead-of triggers for views and system-event triggers for schemas. For more information, see Oracle9i Application Developer's Guide - Fundamentals.
In Oracle Tools
When it contains the PL/SQL engine, an application development tool can process PL/SQL blocks and subprograms. The tool passes the blocks to its local PL/SQL engine. The engine executes all procedural statements at the application site and sends only SQL statements to Oracle. Thus, most of the work is done at the application site, not at the server site.
Furthermore, if the block contains no SQL statements, the engine executes the entire block at the application site. This is useful if your application can benefit from conditional and iterative control.
Frequently, Oracle Forms applications use SQL statements merely to test the value of field entries or to do simple computations. By using PL/SQL instead, you can avoid calls to the Oracle server. Moreover, you can use PL/SQL functions to manipulate field entries.
Advantages of PL/SQL
PL/SQL is a completely portable, high-performance transaction processing language that offers the following advantages:
- Support for SQL
- Support for object-oriented programming
- Better performance
- Higher productivity
- Full portability
- Tight integration with Oracle
- Tight security
Support for SQL
SQL has become the standard database language because it is flexible, powerful, and easy to learn. A few English-like commands such as , , , and make it easy to manipulate the data stored in a relational database.
SQL is non-procedural, meaning that you can state what you want done without stating how to do it. Oracle determines the best way to carry out your request. There is no necessary connection between consecutive statements because Oracle executes SQL statements one at a time.
PL/SQL lets you use all the SQL data manipulation, cursor control, and transaction control commands, as well as all the SQL functions, operators, and pseudocolumns. So, you can manipulate Oracle data flexibly and safely. Also, PL/SQL fully supports SQL datatypes. That reduces the need to convert data passed between your applications and the database.
PL/SQL also supports dynamic SQL, an advanced programming technique that makes your applications more flexible and versatile. Your programs can build and process SQL data definition, data control, and session control statements "on the fly" at run time.
Support for Object-Oriented Programming
Object types are an ideal object-oriented modeling tool, which you can use to reduce the cost and time required to build complex applications. Besides allowing you to create software components that are modular, maintainable, and reusable, object types allow different teams of programmers to develop software components concurrently.
By encapsulating operations with data, object types let you move data-maintenance code out of SQL scripts and PL/SQL blocks into methods. Also, object types hide implementation details, so that you can change the details without affecting client programs.
In addition, object types allow for realistic data modeling. Complex real-world entities and relationships map directly into object types. That helps your programs better reflect the world they are trying to simulate.
Without PL/SQL, Oracle must process SQL statements one at a time. Each SQL statement results in another call to Oracle and higher performance overhead. In a networked environment, the overhead can become significant. Every time a SQL statement is issued, it must be sent over the network, creating more traffic.
However, with PL/SQL, an entire block of statements can be sent to Oracle at one time. This can drastically reduce communication between your application and Oracle. As Figure 1-5 shows, if your application is database intensive, you can use PL/SQL blocks and subprograms to group SQL statements before sending them to Oracle for execution.
PL/SQL stored procedures are compiled once and stored in executable form, so procedure calls are quick and efficient. Also, stored procedures, which execute in the server, can be invoked over slow network connections with a single call. That reduces network traffic and improves round-trip response times. Executable code is automatically cached and shared among users. That lowers memory requirements and invocation overhead.
Figure 1-5 PL/SQL Boosts Performance
Text description of the illustration pls81005_plsql_boosts_performance.gif
PL/SQL also improves performance by adding procedural processing power to Oracle tools. Using PL/SQL, a tool can do any computation quickly and efficiently without calling on the Oracle server. This saves time and reduces network traffic.
PL/SQL adds functionality to non-procedural tools such as Oracle Forms and Oracle Reports. With PL/SQL in these tools, you can use familiar procedural constructs to build applications. For example, you can use an entire PL/SQL block in an Oracle Forms trigger. You need not use multiple trigger steps, macros, or user exits. Thus, PL/SQL increases productivity by putting better tools in your hands.
Also, PL/SQL is the same in all environments. As soon as you master PL/SQL with one Oracle tool, you can transfer your knowledge to other tools, and so multiply the productivity gains. For example, scripts written with one tool can be used by other tools.
Applications written in PL/SQL are portable to any operating system and platform on which Oracle runs. In other words, PL/SQL programs can run anywhere Oracle can run; you need not tailor them to each new environment. That means you can write portable program libraries, which can be reused in different environments.
Tight Integration with SQL
The PL/SQL and SQL languages are tightly integrated. PL/SQL supports all the SQL datatypes and the non-value . That allows you manipulate Oracle data easily and efficiently. It also helps you to write high-performance code.
The and attributes further integrate PL/SQL with SQL. For example, you can use the attribute to declare variables, basing the declarations on the definitions of database columns. If a definition changes, the variable declaration changes accordingly the next time you compile or run your program. The new definition takes effect without any effort on your part. This provides data independence, reduces maintenance costs, and allows programs to adapt as the database changes to meet new business needs.
PL/SQL stored procedures enable you to partition application logic between the client and server. That way, you can prevent client applications from manipulating sensitive Oracle data. Database triggers written in PL/SQL can disable application updates selectively and do content-based auditing of user inserts.
Furthermore, you can restrict access to Oracle data by allowing users to manipulate it only through stored procedures that execute with their definer's privileges. For example, you can grant users access to a procedure that updates a table, but not grant them access to the table itself.