4.4 Variable Declarations

Chapter 4
Variables and Program Data

 

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Before you can make a reference to a variable, you must declare it. (The only exception to this rule is for the index variables of FOR loops.) All declarations must be made in the declaration section of your anonymous block, procedure, function, or package (see Chapter 15, Procedures and Functions, for more details on the structure of the declaration section).

When you declare a variable, PL/SQL allocates memory for the variable's value and names the storage location so that the value can be retrieved and changed. The declaration also specifies the datatype of the variable; this datatype is then used to validate values assigned to the variable.

The basic syntax for a declaration is:

<variable_name> <datatype> [optional default assignment];

where <variable_name> is the name of the variable to be declared and <datatype> is the datatype or subtype which determines the type of data which can be assigned to the variable. The [optional default assignment] clause allows you to initialize the variable with a value, a topic covered in the next section.

4.4.1 Constrained Declarations

The datatype in a declaration can either be constrained or unconstrained. A datatype is constrained when you specify a number which constrains or restricts the magnitude of the value which can be assigned to that variable. A datatype is unconstrained when there are no such restrictions.

Consider the datatype NUMBER. It supports up to 38 digits of precision -- and uses up the memory needed for all those digits. If your variable does not require this much memory, you could declare a number with a constraint, such as the following:

itty_bitty_# NUMBER(1); large_but_constrained_# NUMBER(20,5);

Constrained variables require less memory than unconstrained number declarations like this:

no_limits_here NUMBER;