How does a WITH clause work in PostgreSQL?

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In PostgreSQL, the WITH clause allows you to define one or more subqueries that can be referenced by name in the main query. This is often called a common table expression (CTE) because it’s essentially a temporary result set that you can reference within your main query.

Here’s an example of how the WITH clause can be used:

WITH temp AS (
   SELECT * FROM users WHERE user_id = 1
)
SELECT * FROM temp WHERE user_name = 'John Doe';

In this example, the WITH clause defines a subquery named temp that selects all rows from the users table where the user_id is 1. The main query then selects all rows from the temp table where the user_name is John Doe.

The advantage of using the WITH clause is that it can make complex queries more readable by breaking them up into smaller, more manageable parts. It can also improve query performance by allowing the database to reuse the results of the subquery in the main query.

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