SQLite WITH clause

The SQLite WITH clause is used to specify temporary tables that exist for the duration of a query. These temporary tables can be used to store intermediate results and improve performance.

For example, consider a query that needs to compute the sum of two columns from a table:

SELECT A + B FROM MyTable;

This query can be rewritten using a WITH clause to improve performance:

WITH TempTable AS (
SELECT A, B FROM MyTable
)
SELECT SUM(A) + SUM(B) FROM TempTable;

In this example, the temporary table TempTable is created and populated with the data from MyTable. The query is then rewritten to compute the sum of the columns A and B from TempTable, which is much faster than computing the sum directly from MyTable.

The SQLite WITH clause can be used to create multiple temporary tables. For example:

WITH TempTable1 AS (
SELECT A, B FROM MyTable1
),
TempTable2 AS (
SELECT C, D FROM MyTable2
)
SELECT SUM(A) + SUM(B) + SUM(C) + SUM(D) 
FROM TempTable1, TempTable2;

This query creates two temporary tables, TempTable1 and TempTable2, and then computes the sum of the columns A, B, C, and D from both tables.

The SQLite WITH clause can also be used with recursive queries. For example:

WITH RECURSIVE Fibonacci(n, a, b) AS (
SELECT 0, 0, 1
UNION ALL
SELECT n+1, b, a+b
FROM Fibonacci
WHERE n < 10
)
SELECT * FROM Fibonacci;

This query computes the first 10 numbers in the Fibonacci sequence.

The SQLite WITH clause is an important tool for improving the performance of SQLite queries. When used correctly, it can dramatically speed up the execution of complex SQL queries.