Functions in PL/SQL

A function can be used as a part of SQL expression i.e. we can use them with select/update/merge commands. One most important characteristic of a function is that unlike procedures, it must return a value.

Syntax to create a function:

CREATE [OR REPLACE] FUNCTION function_name 
[(parameter_name  type [, ...])] 

// this statement  is must for functions 
RETURN return_datatype  
{IS | AS} 

BEGIN 
    // program code

[EXCEPTION
   exception_section;

END [function_name];

Example:

1. We have to find the total strength of students using functions present
   in different sections in a school
   
   

   
   
   

   // first lets create a table create table section(s_id int, s_name varchar(20), strength int );    // Inserting values into table insert into section values(1, 'computer science', 20); insert into section values(2, 'portal', 45); insert into section values(3, 'geeksforgeeks', 60);    // Defining function create or replace function totalStrength    // Defining return type return integer as total integer:=0;    begin                            // calculating the sum and storing it in total  select sum(strength) into total from section; return total;    // closing function end totalStrength;    set serveroutput on;    declare answer integer;    begin answer:=totalstrength();    dbms_output.put_line('Total strength of students is  ' || answer);   end;

   Output:

   Total strength of students is  125
   

2. Now, let’s take an example to demonstrate Declaring, Defining and Invoking a simple PL/SQL
   function which will compute and return the reverse of a number.
   
   

   
   
   

   set serveroutput on; declare         a int;     c int;     n int;    rev int:=0;     r int;    // Defining function  function reverse_it( x  IN int)  return  int as z int;    // function code begin     n := x;                while (n > 0)     loop         r := mod(n, 10);         rev := (rev * 10) + r;         n := trunc(n / 10);     end loop;            z := rev;     return z;        end  ;      BEGIN      a := 123456789;              c := reverse_it(a);      dbms_output.put_line('the reverse of number is  ' || c);      END;

   Output:

   the reverse of number is  987654321
   

3. Lets implement a recursive function to calculate the factorial of a number
   Recursive functions example:
   
   

   
   
   

   DECLARE      num int;      answer int;      // Defining function   FUNCTION factorial(x number)   RETURN int    IS      f int;      BEGIN      IF x = 0 THEN         f := 1;      ELSE         f := x * factorial(x - 1);      END IF;   RETURN f;   END;        BEGIN      num := 5;      answer := factorial(num);      dbms_output.put_line(' Factorial of  '|| num || ' is ' || answer);   END;

   Output:

    Factorial of  5 is 120
   

4. Exception handling can be done using exception block in functions but exception handling using try-catch block cannot be done.

   Example:

   
   

   
   
   

   set serveroutput on;    declare a int; b float; myexp exception;    function  sqroot(x int) return float    as answer float;    begin    if x < 0 then raise myexp;    // pre-defined sqrt()  to  // calculate square root  else answer := SQRT(x);     end if; return answer;    exception when myexp then dbms_output.put_line('square of negative number is                        not allowed so returning the same                       number'); return x; end;    begin b := sqroot(-2); dbms_output.put_line('the value is '|| b); end;

   Output:

   square of negative number is  not allowed so returning the same number
   the value is -2
   
   

Advantages:

1. We can make a single call to the database to run a block of statements thus it improves the performance against running SQL multiple times. This will reduce the number of calls between the database and the application.
2. We can divide the overall work into small modules which becomes quite manageable also enhancing the readability of the code.
3. It promotes reusability.
4. It is secure since the code stays inside the database thus hiding internal database details from the application(user). The user only makes a call to the PL/SQL functions. Hence security and data hiding is ensured.