Declarations in SQL
Your program stores values in the variables and constants. As the program executes, the value of the variables can change, but the values constants cannot.
You can declare the variables and constants in the declarative part of any PL/SQL block, package, or subprogram. The Declarations allocate the storage space for a value, state its datatype, and name the storage location and hence, you can reference it.
A couple of examples are shown below:
birthday DATE;
emp_count SMALLINT := 0;
The first declaration names a variable of the type DATE. The second declaration names a variable of the type SMALLINT and uses the assignment operator to assign an initial value of zero to the variable.
The example next show that the expression following the assignment operator can be arbitrarily complex and can refer to the earlier initialized variables:
pi REAL := 3.14159;
radius REAL := 1;
area REAL := pi * radius**2;
By default, the variables are initialized to NULL. So, these declarations are equal:
birthday DATE;
birthday DATE := NULL;
In the declaration of a constant, the keyword CONSTANT should precede the type of the specifier, as the example below shows:
credit_limit CONSTANT REAL := 5000.00;
This declaration names a constant of the type REAL and assigns an initial value of 5000 to the constant. The constant must be initialized in its declaration. Or else, you get a compilation error whenever the declaration is elaborated. (The procedure of a declaration by the PL/SQL compiler is known as the elaboration.)