Declaring Variables in PostgreSQL Queries

In PostgreSQL, declaring variables can be a bit different than in other SQL databases. Unlike some other databases, PostgreSQL does not support declaring variables directly in SQL queries using a DECLARE statement like MS SQL Server. However, there are alternative methods to achieve similar functionality.

Using PL/pgSQL

One way to declare variables is by using PL/pgSQL, which is a procedural language for PostgreSQL. You can use the DO command to execute a block of PL/pgSQL code that declares and uses variables. Here’s an example:

DO $$
DECLARE myvar integer;
BEGIN
    SELECT 5 INTO myvar;
    
    RAISE NOTICE 'myvar: %', myvar;
END $$;

In this example, myvar is declared as an integer variable, assigned the value 5, and then its value is printed to the console using RAISE NOTICE.

Using Temporary Tables or Common Table Expressions (CTEs)

Another approach is to use temporary tables or CTEs to store values that can be used in place of variables. For example:

WITH myconstants (var1, var2) as (
   VALUES (5, 'foo')
)
SELECT *
FROM somewhere, myconstants
WHERE something = var1
   OR something_else = var2;

This method allows you to define temporary values that can be used in your query.

Using Dynamic Config Settings

PostgreSQL also provides dynamic config settings that can be used as variables. You can set a config setting using the SET command and then retrieve its value using the current_setting() function:

SET session my.vars.id = '1';

SELECT *
FROM person 
WHERE id = current_setting('my.vars.id')::int;

Note that config settings are always stored as strings, so you need to cast them to the correct data type when using them.

Using psql Client Variables

If you’re using the psql client, you can use its built-in variable feature. You can set a variable using the \set command and then reference it in your query:

my_db=> \set myvar 5
my_db=> SELECT :myvar + 1 AS my_var_plus_1;

Keep in mind that this method only works in the psql client and not in other SQL clients.

In summary, while PostgreSQL does not support declaring variables directly in SQL queries like some other databases, there are alternative methods to achieve similar functionality using PL/pgSQL, temporary tables or CTEs, dynamic config settings, or psql client variables.