PL/SQL User's Guide and Reference

Actual versus Formal Parameters

Subprograms pass information using parameters. The variables or expressions referenced in the parameter list of a subprogram call are actual parameters. For example, the following procedure call lists two actual parameters named emp_num and amount:

raise_salary(emp_num, amount);

The next procedure call shows that in some cases, expressions can be used as actual parameters:

raise_salary(emp_num, merit + cola);

The variables declared in a subprogram specification and referenced in the subprogram body are formal parameters. For example, the following procedure declares two formal parameters named emp_id and increase:

PROCEDURE raise_salary (emp_id INTEGER, increase REAL) IS
   current_salary  REAL;
   ...
BEGIN
   SELECT sal INTO current_salary FROM emp WHERE empno = emp_id;
   ...
   UPDATE emp SET sal = sal + increase WHERE empno = emp_id;
END raise_salary;

Though not necessary, it is good programming practice to use different names for actual and formal parameters.

When you call procedure raise_salary, the actual parameters are evaluated and the result values are assigned to the corresponding formal parameters. Before assigning the value of an actual parameter to a formal parameter, PL/SQL converts the datatype of the value if necessary. For example, the following call to raise_salary is legal:

raise_salary(emp_num, '2500');

The actual parameter and its corresponding formal parameter must have compatible datatypes. For instance, PL/SQL cannot convert between the DATE and REAL datatypes. Also, the result value must be convertible to the new datatype. The following procedure call raises the predefined exception VALUE_ERROR because PL/SQL cannot convert the second actual parameter to a number:

raise_salary(emp_num, '$2500');  -- note the dollar sign

For more information, see "Datatype Conversion" .