PL/SQL Procedures, Functions and Packages

What is a Stored Procedure?

A stored procedure or in simple a proc is a named PL/SQL block which performs one or more specific task. This is similar to a procedure in other programming languages.

A procedure has a header and a body. The header consists of the name of the procedure and the parameters or variables passed to the procedure. The body consists or declaration section, execution section and exception section similar to a general PL/SQL Block.

A procedure is similar to an anonymous PL/SQL Block but it is named for repeated usage.

We can pass parameters to procedures in three ways.
1) IN-parameters
2) OUT-parameters
3) IN OUT-parameters

A procedure may or may not return any value.

General Syntax to create a procedure is:

CREATE [OR REPLACE] PROCEDURE proc_name [list of parameters]

IS   

   Declaration section

BEGIN   

   Execution section

EXCEPTION   

  Exception section

END;

IS - marks the beginning of the body of the procedure and is similar to DECLARE in anonymous PL/SQL Blocks. The code between IS and BEGIN forms the Declaration section.

The syntax within the brackets [ ] indicate they are optional. By using CREATE OR REPLACE together the procedure is created if no other procedure with the same name exists or the existing procedure is replaced with the current code.

The below example creates a procedure ‘employer_details’ which gives the details of the employee.

1> CREATE OR REPLACE PROCEDURE employer_details

2> IS

3>  CURSOR emp_cur IS

4>  SELECT first_name, last_name, salary FROM emp_tbl;

5>  emp_rec emp_cur%rowtype;

6> BEGIN

7>  FOR emp_rec in sales_cur

8>  LOOP

9>  dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name

10>    || ' ' ||emp_cur.salary);

11> END LOOP;

12>END;

13> /

How to execute a Stored Procedure?

There are two ways to execute a procedure.

1) From the SQL prompt.

 EXECUTE [or EXEC] procedure_name;

2) Within another procedure – simply use the procedure name.

  procedure_name;

NOTE: In the examples given above, we are using backward slash ‘/’ at the end of the program. This indicates the oracle engine that the PL/SQL program has ended and it can begin processing the statements.

What is a Function in PL/SQL?

A function is a named PL/SQL Block which is similar to a procedure. The major difference between a procedure and a function is, a function must always return a value, but a procedure may or may not return a value.

General Syntax to create a function is

CREATE [OR REPLACE] FUNCTION function_name [parameters] RETURN return_datatype;  IS  Declaration_section  BEGIN  Execution_section Return return_variable;  EXCEPTION  exception section  Return return_variable;  END;

1) Return Type: The header section defines the return type of the function. The return datatype can be any of the oracle datatype like varchar, number etc.
2) The execution and exception section both should return a value which is of the datatype defined in the header section.

For example, let’s create a frunction called ''employer_details_func' similar to the one created in stored proc

1> CREATE OR REPLACE FUNCTION employer_details_func

2>    RETURN VARCHAR(20);

3> IS

5>    emp_name VARCHAR(20);

6> BEGIN

7>      SELECT first_name INTO emp_name

8>      FROM emp_tbl WHERE empID = '100';

9>      RETURN emp_name;

10> END;

11> /

In the example we are retrieving the ‘first_name’ of employee with empID 100 to variable ‘emp_name’.
The return type of the function is VARCHAR which is declared in line no 2. 
The function returns the 'emp_name' which is of type VARCHAR as the return value in line no 9.

How to execute a PL/SQL Function?

A function can be executed in the following ways.

1) Since a function returns a value we can assign it to a variable.

employee_name :=  employer_details_func;

If ‘employee_name’ is of datatype varchar we can store the name of the employee by assigning the return type of the function to it.

2) As a part of a SELECT statement

SELECT employer_details_func FROM dual;

3) In a PL/SQL Statements like,

dbms_output.put_line(employer_details_func);

This line displays the value returned by the function.

How to pass parameters to Procedures and Functions in PL/SQL?

In PL/SQL, we can pass parameters to procedures and functions in three ways.

1) IN type parameter: These types of parameters are used to send values to stored procedures. 
2) OUT type parameter: These types of parameters are used to get values from stored procedures. This is similar to a return type in functions.
3) IN OUT parameter: These types of parameters are used to send values and get values from stored procedures.

NOTE: If a parameter is not explicitly defined a parameter type, then by default it is an IN type parameter.

1) IN parameter: This is similar to passing parameters in programming languages. We can pass values to the stored procedure through these parameters or variables. This type of parameter is a read only parameter. We can assign the value of IN type parameter to a variable or use it in a query, but we cannot change its value inside the procedure. General syntax to pass a IN parameter is

CREATE [OR REPLACE] PROCEDURE procedure_name (

  param_name1 IN datatype, param_name12 IN datatype ... )

• param_name1, param_name2... are unique parameter names.
• datatype - defines the datatype of the variable.
• IN - is optional, by default it is a IN type parameter.

2) OUT Parameter:

The OUT parameters are used to send the OUTPUT from a procedure or a function. This is a write-only parameter i.e, we cannot pass values to OUT paramters while executing the stored procedure, but we can assign values to OUT parameter inside the stored procedure and the calling program can recieve this output value.

The General syntax to create an OUT parameter is

CREATE [OR REPLACE] PROCEDURE proc2 (param_name OUT datatype)

The parameter should be explicity declared as OUT parameter. 

3) IN OUT Parameter:

The IN OUT parameter allows us to pass values into a procedure and get output values from the procedure. This parameter is used if the value of the IN parameter can be changed in the calling program.

By using IN OUT parameter we can pass values into a parameter and return a value to the calling program using the same parameter. But this is possible only if the value passed to the procedure and output value have a same datatype. This parameter is used if the value of the parameter will be changed in the procedure.

The General syntax to create an IN OUT parameter is

CREATE [OR REPLACE] PROCEDURE proc3 (param_name IN OUT datatype)

The below examples show how to create stored procedures using the above three types of parameters.

Example1:

Using IN and OUT parameter:

Let’s create a procedure which gets the name of the employee when the employee id is passed.

1> CREATE OR REPLACE PROCEDURE emp_name (id IN NUMBER, emp_name OUT NUMBER)

2> IS

3> BEGIN

4>    SELECT first_name INTO emp_name

5>    FROM emp_tbl WHERE empID = id;

6> END;

7> /

We can call the procedure ‘emp_name’ in this way from a PL/SQL Block.

1> DECLARE

2>  empName varchar(20);

3>  CURSOR id_cur SELECT id FROM emp_ids;

4> BEGIN

5> FOR emp_rec in id_cur

6> LOOP

7>   emp_name(emp_rec.id, empName);

8>   dbms_output.putline('The employee ' || empName || ' has id ' || emp-rec.id);

9> END LOOP;

10> END;

11> /

In the above PL/SQL Block 
In line no 3; we are creating a cursor ‘id_cur’ which contains the employee id.
In line no 7; we are calling the procedure ‘emp_name’, we are passing the ‘id’ as IN parameter and ‘empName’ as OUT parameter.
In line no 8; we are displaying the id and the employee name which we got from the procedure ‘emp_name’.

Example 2:

Using IN OUT parameter in procedures:

1> CREATE OR REPLACE PROCEDURE emp_salary_increase

2> (emp_id IN emptbl.empID%type, salary_inc IN OUT emptbl.salary%type)

3> IS

4>    tmp_sal number;

5> BEGIN

6>    SELECT salary

7>    INTO tmp_sal

8>    FROM emp_tbl

9>    WHERE empID = emp_id;

10>   IF tmp_sal between 10000 and 20000 THEN

11>      salary_inout := tmp_sal * 1.2;

12>   ELSIF tmp_sal between 20000 and 30000 THEN

13>      salary_inout := tmp_sal * 1.3;

14>   ELSIF tmp_sal > 30000 THEN

15>      salary_inout := tmp_sal * 1.4;

16>   END IF;

17> END;

18> /

The below PL/SQL block shows how to execute the above 'emp_salary_increase' procedure.

1> DECLARE

2>    CURSOR updated_sal is

3>    SELECT empID,salary

4>    FROM emp_tbl;

5>    pre_sal number;

6> BEGIN

7>   FOR emp_rec IN updated_sal LOOP

8>       pre_sal := emp_rec.salary;

9>       emp_salary_increase(emp_rec.empID, emp_rec.salary);

10>       dbms_output.put_line('The salary of ' || emp_rec.empID ||

11>                ' increased from '|| pre_sal || ' to '||emp_rec.salary);

12>   END LOOP;

13> END;

14> /