PostgreSQL Tutorial: PL/pgSQL IF Statement

August 4, 2023

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

Table of Contents

Introduction to PL/pgSQL IF statement

The if statement determines which statements to execute based on the result of a boolean expression.

PL/pgSQL provides you with three forms of the if statements.

if then
if then else
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 if a condition is true. If the condition evaluates to false, the control is passed to the next statement after the END if part.

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

The statements can be one or more statements that will be executed if the condition is true. It can be any valid statement, even another if statement.

When an if statement is placed inside another if statement, it is called a nested-if statement.

The following flowchart illustrates the simple if statement.

PL/pgSQL if statement

See the following example:

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 $$;

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

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

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

ERROR:  The film 0 could not be found

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 following illustrates the syntax of the if-then-else statement:

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

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.

The following flowchart illustrates the if else statement.

PL/pgSQL if else statement

See the following example:

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 $$;

In this example, the film id 100 exists in the film table so that the FOUND variable was set to true. Therefore, the statement in the else branch executed.

Here is the output:

NOTICE:  The film title is Brooklyn Desert

3) PL/pgSQL if-then-elsif Statement

The following illustrates 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;

The if and if then else statements evaluate one condition. However, the if then elsif statement evaluates multiple conditions.

If a condition is true, the corresponding statement in that branch is executed.

For example, if the condition_1 is true then the if then elsif executes the statement_1 and stops evaluating the other conditions.

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

The following flowchart illustrates the if then elsif statement:

PL/pgSQL if ELSif else 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 $$;

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 the film a description based on the length of the film.

Summary

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