Organize complex SQL queries with Common Table Expressions

Complex SQL database queries often contain subqueries.

   WHERE name IN (SELECT name 
         FROM ... 
         WHERE ...)

These can quickly become unreadable, especially if multiple subqueries are involved. A nice way to organise such queries with multiple subqueries is called Common Table Expressions (CTE), or colloquially: “WITH queries”. They are supported by most contemporary SQL databases.

When using Common Table Expressions you list all the subqueries after a WITH keyword in front of the actual query and assign a name to each subquery:

  <subquery1_name> AS (SELECT ...),
  <subquery2_name> AS (SELECT ...),
  [ ... ]
FROM ...
[ ... ]

You can now refer to these subqueries in the main query by their names. They are conceptually just like temporary views. You can also refer to a subquery in the main query multiple times, whereas if the subquery was inlined you would have to repeat it. A subquery defined as a Common Table Expression can also refer to the preceding subqueries in the subquery list.


A Common Table Expression can even refer to itself, which is a recursive definition. In some database systems you have to add the RECURSIVE keyword after WITH, in others you can leave it out. Here’s a recursive CTE that calculates factorial numbers:

WITH RECURSIVE factorials (n, fact) AS 
  (SELECT 0, 1
   SELECT n+1, (n+1)*fact FROM factorials
          WHERE n < 6)
SELECT * FROM factorials;
0    1
1    1
2    2
3    6
4   24
5  120

You could also use such a recursive query to descend into a tree hierarchy.

While the use cases for recursive queries are less frequent, I find the general concept of Common Table Expressions very useful to make complex queries more readable.