PL/pgSQL Loop Statements

Summary: in this tutorial, you will learn about the PL/pgSQL loop statement that executes a block of code repeatedly.

Introduction to PL/pgSQL Loop statement

The loop defines an unconditional loop that executes a block of code repeatedly until terminated by an exit or return statement.

The following illustrates the syntax of the loop statement:

<<label>>
loop
   statements;
end loop;

Typically, you use an if statement to terminate the loop based on a condition like this:

<<label>>
loop
   statements;
   if condition then
      exit;
   end if;
end loop;

The exit statement terminates the loop immediately.

It’s possible to place a loop statement inside another loop statement. When a loop statement is placed inside another loop statement, it is called a nested loop:

<<outer>>
loop
   statements;
   <<inner>>
   loop
     /* ... */
     exit <<inner>>
   end loop;
end loop;

When you have nested loops, it’s necessary to use loop labels. The loop labels allow you to specify the loop in the exit and continue statements, indicating which loop these statements refer to.

PL/pgSQL loop statement examples

Let’s explore some examples of using the loop statement.

1) Basic PL/pgSQL loop example

The following example uses a loop statement to display five numbers from 1 to five:

do $$

declare
    counter int := 0;
begin

  loop
  	counter = counter + 1;
	raise notice '%', counter;

	if counter = 5 then
		exit;
	end if;

  end loop;

end;

$$;

Output:

NOTICE:  1
NOTICE:  2
NOTICE:  3
NOTICE:  4
NOTICE:  5

How it works.

First, declare a variable counter and initialize its value to zero:

counter int := 0;

Second, increase the value of the counter variable by one in each iteration of the loop:

counter = counter + 1;

Third, display the current value of the counter:

raise notice '%', counter;

Finally, terminate the loop if the current value of the counter variable is 5:

if counter = 5 then
   exit;
end if;

Since the initial value of the counter is zero, the loop executes five times before it is terminated.

In practice, you can combine the if and exit statements into a single statement like this:

exit when counter = 5;

For example:

do $$

declare
    counter int := 0;
begin

  loop
  	counter = counter + 1;
	raise notice '%', counter;
	exit when counter = 5;
  end loop;

end;

$$;

Note that you will learn more about the exit statement in the upcoming tutorial.

2) Using a loop with a label

The following example illustrates how to use a loop label:

do $$

declare
    counter int := 0;
begin

 <<my_loop>>
  loop
  	counter = counter + 1;
	raise notice '%', counter;
	exit my_loop when counter = 5;
  end loop;

end;

$$;

Output:

NOTICE:  1
NOTICE:  2
NOTICE:  3
NOTICE:  4
NOTICE:  5

How it works.

In this example, we place a loop label my_loop inside <<>> before the LOOP keyword.

Inside the loop’s body, we reference the loop label (my_loop) in the exit statement to explicitly instruct PostgreSQL to terminate the loop specified by the loop label: my_loop.

It’ll be more practical to use a loop label when you have a nested loop.

3) Nested loop example

The following example illustrates how to use a nested loop with labels:

do $$

declare
	row_var int := 0;
	col_var int := 0;
begin
	<<outer_loop>>
	loop
		row_var = row_var + 1;
		<<inner_loop>>
		loop
			col_var = col_var + 1;
			raise notice '(%, %)', row_var, col_var;

			-- terminate the inner loop
			exit inner_loop when col_var = 3;
		end loop;
		-- reset the column
		col_var = 0;

		-- terminate the outer loop
		exit outer_loop when row_var = 3;
	end loop;
end;

$$;

Output:

NOTICE:  (1, 1)
NOTICE:  (1, 2)
NOTICE:  (1, 3)
NOTICE:  (2, 1)
NOTICE:  (2, 2)
NOTICE:  (2, 3)
NOTICE:  (3, 1)
NOTICE:  (3, 2)
NOTICE:  (3, 3)

How it works.

First, declare two variables row_var and col_var, and initialize their values to zero:

row_var int := 0;
col_var int := 0;

Second, use the outer_loop as the label for the outer loop. In the outer loop, increase the value of the row_var by one, execute the nested loop, and reset the col_var in each iteration.

If the row_var is 3, exit the outer loop by referencing the outer_loop label in the exit statement.

Third, use the inner_loop as the label for the inner loop. In the inner loop, increase the value of col_var by one, display the current values of row_var and col_var variables, and terminate the inner loop when the value of col_var is 3.

Summary

Use the PL/pgSQL LOOP statement to create unconditional loops.
The loop can be nested.
Use the exit statement to terminate a loop prematurely.