PL SQL Block Structures

What is PL SQL

• A good way to get acquainted with PL/SQL is to look at a sample program.

• PL/SQL combines the data manipulating power of SQL with the data processing power of procedural languages.

Block Structure

• PL/SQL is a block-structured language.

• 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.

Logical Blocks

• Each logical block corresponds to a problem or subproblem to be solved.

• A block (or sub-block) lets you group logically related declarations and statements.

• The declarations are local to the block and cease to exist when the block completes.

Declarative

• First comes the declarative part, in which items can be declared.

• Once declared, items can be manipulated in the executable part.

Nesting

• 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.

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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.

SQL Datatypes

• Variables can have any SQL datatype, such as CHAR, DATE, or NUMBER, or any PL/SQL datatype, such as BOOLEAN or BINARY_INTEGER

Assigning Values

• You can assign values to a variable in three ways.

• The first way uses the assignment operator (:=), a colon followed by an equal sign.

• Examples:

tax := price * tax_rate;

valid_id := FALSE;

2nd Way to Assign Values

• The second way to assign values to a variable is by selecting (or fetching) database values into it.

• Example: (Compute a 10% bonus when you select the salary of an employee. )

SELECT sal * 0.10 INTO bonus FROM emp WHERE empno = emp_id;

3rd Way to Assign a Value

• The third way to assign values to a variable is by passing it as an OUT or IN OUT parameter to a subprogram.

• Example:

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 Constants

• Declaring a constant is like declaring a variable except that you must add the keyword CONSTANT and immediately assign a value to the constant. Thereafter, no more assignments to the constant are allowed.

Example:

credit_limit CONSTANT REAL := 5000.00;

Cursors

• Oracle uses work areas to execute SQL statements and store processing information.

2 types of Cursors

• 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.

Implicit Cursor

• PL/SQL implicitly declares a cursor for all SQL data manipulation statements, including queries that return only one row.

Explicit Cursor

• For queries that return more than one row, you can explicitly declare a cursor to process the rows individually.

• Example:

DECLARE CURSOR c1 IS SELECT empno, ename, job FROM emp WHERE deptno = 20;

Cursor For Loops

• A cursor FOR loop implicitly declares its loop index as a record that represents a row fetched from the database.

DECLARE CURSOR c1 IS SELECT ename, sal, hiredate, deptno FROM emp;

Fetch

• Next, it opens a cursor, repeatedly fetches rows of values from the result set into fields in the record.

BEGIN FOR emp_rec IN c1 LOOP ... salary_total := salary_total + emp_rec.sal;

Closes Loop

• Then closes the cursor when all rows have been processed.

END LOOP;

Full Example of Cursor For Loop

DECLARE CURSOR c1 IS SELECT ename, sal, hiredate, deptnoFROM emp;

... BEGIN

FOR emp_rec IN c1 LOOP ... salary_total := salary_total + emp_rec.sal;END LOOP;

Cursor Variable

• A cursor variable points to the current row in the result set of a multi-row query.

• 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.

Example for Cursor Variable• The following procedure opens the cursor variable generic_cv 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;

• The program below processes an order for a tennis racket.

DECLARE

• First, it declares a variable of type NUMBER to store the quantity of tennis rackets on hand.

DECLARE qty_on_hand NUMBER(5);

• Then, it retrieves the quantity on hand from a database table named inventory.

BEGIN SELECT quantity INTO qty_on_hand FROM inventory WHERE product = 'TENNIS RACKET' FOR UPDATE OF quantity;

(condition) IF THEN

• If the quantity is greater than zero, the program updates the table and inserts a purchase record into another table named purchase_record.

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

• Otherwise, the program inserts an out-of-stock record into the purchase_record table.

ELSE INSERT INTO purchase_record

VALUES ('Out of tennis rackets', SYSDATE); END IF; COMMIT; END;