Common Table Expressions

In SQL, a common table expression (CTE) is a named temporary result set that you can reference within a SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. CTEs are useful for organizing and simplifying complex queries, and can be used to improve the readability and maintainability of your SQL code.

A CTE is defined using the WITH clause, which is followed by a comma-separated list of CTE definitions. Each CTE definition consists of a CTE name and a query that defines the result set for the CTE. Here is an example of a simple CTE:

WITH cte_name AS (
  SELECT column1, column2
  FROM table1
  WHERE condition
)
SELECT *
FROM cte_name;

In this example, the CTE named "cte_name" is defined as a SELECT statement that retrieves data from the "table1" table. The CTE is then referenced in the outer SELECT statement to retrieve all rows from the CTE result set.

CTEs can also be used to define recursive queries, which are queries that reference themselves to retrieve data from a hierarchical structure. Here is an example of a recursive CTE:

WITH cte_name (column1, column2, level) AS (
  SELECT column1, column2, 1 as level
  FROM table1
  WHERE condition
  UNION ALL
  SELECT t.column1, t.column2, c.level + 1
  FROM cte_name c
  JOIN table1 t ON t.parent_id = c.id
)
SELECT *
FROM cte_name;

In this example, the CTE is defined with a UNION ALL clause that combines the results of the initial SELECT statement with the results of a recursive SELECT statement that references the CTE itself. This allows the CTE to retrieve data from the "table1" table and all of its child rows in a hierarchical structure.

CTEs can be used in a variety of scenarios to simplify and improve the readability of your SQL code. They are especially useful for organizing and refactoring complex queries, and can be a valuable tool for improving the maintainability of your database applications.