PL/pgSQL IF Statement

Summary: in this tutorial, you will learn how to use the PL/pgSQL if statements to execute a command based on a specific condition.

Introduction to PL/pgSQL IF Statement

The if statement allows you to execute one or more statements based on a condition. PL/pgSQL provides you with three forms of the if statements:

1. if then
2. if then else
3. if then elsif

1) PL/pgSQL if-then statement

The following illustrates the simplest form of the if statement:

if condition then
   statements;
end if;

The if statement executes statements when a condition is true. If the condition evaluates to false, the control is passed to the next statement after the end if .

The condition is a boolean expression that evaluates to true or false.

The statements can be one or more statements that you want to execute when the condition is true. It may contain other if statements.

When you place an if statement is within another if statement, you’ll have a nested-if statement.

The following flowchart illustrates the simple if statement.

The following example uses an if statement to check if a query returns any rows:

do $$
declare
  selected_film film%rowtype;
  input_film_id film.film_id%type = 0;
begin

  select * from film
  into selected_film
  where film_id = input_film_id;

  if not found then
     raise notice'The film % could not be found',
	    input_film_id;
  end if;
end $$;

Output:

NOTICE:  The film 0 could not be found

In this example, we select a film by a specific film id (0).

The found is a global variable that is available in PL/pgSQL. The select into statement sets the found variable if a row is assigned or false if no row is returned.

We use the if statement to check if the film with id (0) exists and raise a notice if it does not.

if not found then
   raise notice'The film % could not be found', input_film_id;
end if;

If you change the value of the input_film_id variable to some value that exists in the film table like 100, you will not see any message.

2) PL/pgSQL if-then-else statement

The if...then...else statement executes the statements in the if branch if the condition evaluates to true; otherwise, it executes the statements in the else branch.

Here’s the syntax of the if...then...else statement:

if condition then
  statements;
else
  alternative-statements;
end if;

The following flowchart illustrates the if else statement.

The following example uses an if…then…else statement to display a message showing that a film with a specific id exists or not:

do $$
declare
  selected_film film%rowtype;
  input_film_id film.film_id%type := 100;
begin

  select * from film
  into selected_film
  where film_id = input_film_id;

  if not found then
     raise notice 'The film % could not be found',
	    input_film_id;
  else
     raise notice 'The film title is %', selected_film.title;
  end if;
end $$;

Output:

NOTICE:  The film title is Brooklyn Desert

In this example, because the film id 100 exists in the film table the found variable is true. Therefore, the statement in the else branch is executed.

3) PL/pgSQL if-then-elsif Statement

Unlike the if and if...then...else statements that evaluate only one condition, the if then elsif statement allows you to evaluate multiple conditions. and execute one or more statements when a condition is true.

Here’s the syntax of the if...then...elsif statement:

if condition_1 then
  statement_1;
elsif condition_2 then
  statement_2
...
elsif condition_n then
  statement_n;
else
  else-statement;
end if;

In this syntax, if the condition_1 is true then the if...then...elsif statement executes the statement_1 and stops evaluating the other conditions such as condition_2, condition_3, and so on.

If all conditions are evaluated to false, the if...then...elsif executes the statements in the else branch.

The following flowchart illustrates the if then elsif statement:

Let’s look at the following example:

do $$
declare
   v_film film%rowtype;
   len_description varchar(100);
begin

  select * from film
  into v_film
  where film_id = 100;

  if not found then
     raise notice 'Film not found';
  else
      if v_film.length >0 and v_film.length <= 50 then
		 len_description := 'Short';
	  elsif v_film.length > 50 and v_film.length < 120 then
		 len_description := 'Medium';
	  elsif v_film.length > 120 then
		 len_description := 'Long';
	  else
		 len_description := 'N/A';
	  end if;

	  raise notice 'The % film is %.',
	     v_film.title,
	     len_description;
  end if;
end $$;

Output:

NOTICE:  The Brooklyn Desert film is Long.

How it works:

• First, select the film with id 100. If the film does not exist, raise a notice that the film is not found.
• Second, use the if...then....elsif statement to assign a description to a film based on the length of the film.

Summary

• Use the if..then statement to execute one or more statements when a condition is true.
• Use the if...then...else statement to execute statements when a condition is true and execute other statements when the condition is false.
• Use the if...then...elsif statement to evaluate multiple conditions and execute statements when the corresponding condition is true.