Variables and Constants in PL/SQL
The PL/SQL permits you to declare constants and variables, and then use them in SQL and procedural statements anywhere in the expression. Though, forward references are not permitted. So, you should declare a constant or variable before referencing it in another statement, involving other declarative statements.
Declaring Variables
The Variables can have any SQL datatype, like CHAR, DATE, or NUMBER, or any PL/SQL datatype, like BOOLEAN or BINARY_INTEGER. For e.g., suppose that you want to declare a variable name part_no to hold 4-digit numbers and a variable name in_stock to hold the Boolean value TRUE or FALSE. You can declare these variables as shown:
part_noNUMBER(4);
in_stock BOOLEAN;
You can also declare nested tables, records using the TABLE, VARRAY, variable-size arrays (varrays for short), and RECORD composite datatypes.
Assigning Values to a Variable
You can assign values to a variable in 2 ways. The first way uses the assignment operator (:=), a colon followed by an equal sign. You put the variable to the left of the operator and an expression to the right. Some examples are as follow:
tax := price * tax_rate;
bonus := current_salary * 0.10;
amount := TO_NUMBER(SUBSTR('750 dollars', 1, 3));
valid := FALSE;
The second way to assign values to a variable is to fetch or select database values into it. In the example below, you have Oracle calculate a 10% bonus when you select the salary of an employee:
SELECT sal * 0.10 INTO bonus FROM emp WHERE empno = emp_id;
After that, you can use the variable bonus in another calculation or insert its value into a database table.
Declaring Constants
Declaring a constant is such as declaring a variable except that you must add the keyword CONSTANT and right away assign a value to the constant. Subsequently, no more assignments to the constant are allowed. In the example below, you declare a constant named credit_limit:
credit_limit CONSTANT REAL := 5000.00;