Expression:

This is a randomly complex combination of constants, variables, literals, operators, & function calls. The simplest expression consists of a single variable. If the assignment statement is executed, the expression is computed and the resulting value is stored in the assignment target. The value and target must have well-suited datatypes.

By default, unless the variable is initialized in its declaration, it is initialized to NULL every time a subprogram or block is entered. Therefore, never reference a variable before you assign it a value. You can't assign nulls to a variable defined as NOT NULL. If you attempt, the PL/SQL raises the predefined exception VALUE_ERROR. The values TRUE, FALSE, & NULL can also be assigned to a Boolean variable. If applied to an expression, the relational operators return a Boolean value. And hence, the assignment below is legal:

DECLARE

out_of_range BOOLEAN;

...

BEGIN

...

out_of_range := (salary < minimum) OR (salary > maximum);

As the later illustration shows, you can assign the value of an expression to an  explicit field in a record:

DECLARE

emp_rec emp%ROWTYPE;

BEGIN

...

emp_rec.sal := current_salary + increase;

Furthermore, you can assign values to all the fields in a record at once. The PL/SQL permits an aggregate assignment between the whole records if their declarations refer to similar cursor or table. For illustration, the assignment below is legal:

DECLARE

emp_rec1 emp%ROWTYPE;

emp_rec2 emp%ROWTYPE;

dept_rec dept%ROWTYPE;

BEGIN

...

emp_rec1 := emp_rec2;

The later assignment is illegal as you cannot use the assignment operator to assign the list of values to a record:

dept_rec := (60, 'PUBLICITY', 'LOS ANGELES');

Using the syntax below, you can assign the value of an expression to the specific element in a collection:

collection_name(index) := expression;

In the example below, you assign the uppercase value of the last_name to the third row in nested table ename_tab:

ename_tab(3) := UPPER(last_name);