How Do I Declare and Use Variables in PostgreSQL Queries?

PostgreSQL Variable Declaration and Usage

Unlike MS SQL Server, PostgreSQL doesn't declare variables in the same manner. Instead, the WITH clause provides a mechanism for defining and assigning values to variables within a query.

Declaring a PostgreSQL Variable

The syntax for declaring a variable in a PostgreSQL query uses the WITH clause:

<code class="language-sql">WITH <variable_name> AS (<value>)</code>

For instance, to create an integer variable myvar with a value of 5, the syntax is:

<code class="language-sql">WITH myvar AS (SELECT 5)</code>

Note that you must assign a value using a SELECT statement within the AS clause.

Using a Declared Variable

After declaring a variable, you can use it in your query by referencing its name. For example, this query retrieves all rows from the somewhere table where the something column matches the value of myvar:

<code class="language-sql">WITH myvar AS (SELECT 5)
SELECT *
FROM somewhere
WHERE something = (SELECT * FROM myvar);</code>

Remember, the WITH clause must precede the SELECT statement. The variable's value is accessed using a subquery referencing the variable name.

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