Because no “cap” has been placed on this (via a WHERE or LIMIT), this means we’ve defined an infinitely recurring CTE.The result is we’re defining a common table expression named “infinite” to be the union of “a single row” and “all other rows”.And on the right we do “SELECT * FROM infinite” - meaning, select everything currently in the table.Next the “UNION ALL” says “combine the output of what’s on the left, with the output of what’s on the right.“SELECT 1” seeds that CTE’s output with a single row - containing “1”.“WITH RECURSIVE infinite” defines a common table expression named “infinite”.Let’s break down why that query will never finish: Take this example: sqlite> WITH RECURSIVE infinite AS ( This just combines two result sets into one (so long as they have the same number of columns): sqlite> SELECT 1, 2 They key to this is using a “Compound Select Statements”, such as the UNION ALL operator.
But the real power of common table expressions is when you define an expression that recursively selects itself.
So, common table expressions can be used to restructure a query to make it more readable, by moving the subqueries out in front. Sqlite> WITH fooCTE AS (SELECT * FROM foo)Īdditionally, you can define as many CTEs as you want in a single query: sqlite> WITH aCTE AS (SELECT 'a'), Similarly, a CTE can query other tables: sqlite> CREATE TABLE foo ( bar INTEGER ) Such that the final result is a single value: 1īut a CTE can have multiple columns, too, and those columns can be assigned names: sqlite> WITH twoCol( a, b ) AS ( SELECT 1, 2 ).We’ve “filled” it with the output of SELECT 1, which is just 1 row.We’ve defined a common table expression named “one”.Accordingly, the simplest CTE version of the above query would be like: sqlite> WITH one AS ( SELECT 1 ) A “Common Table Expression” is basically the same as a subquery, except assigned a name and defined prior to the query in which it’s referenced. This just selects all the results from the subquery - which in this case, is just a single row. Next, consider the simplest subquery: sqlite> SELECT * FROM ( SELECT 1 ) Here’s my attempt to write a tutorial that starts as simple as possible.įirst, let’s start with the simplest query: sqlite> SELECT 1 Īll this does is return a result set containing a row. I’ve been trying to wrap my head around Common Table Expressions for a while, and all the tutorials I’ve read started out with “simple” examples that were way too advanced for me to follow.
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