A Common Table Expression (CTE) is a temporary result set in a SELECT, INSERT, UPDATE, or DELETE statement that you can reference within the context of another SQL statement. CTEs provide a way to define complex queries and make SQL code more readable and modular.

Here’s a breakdown of the key components and usage of SQL CTE:


The basic syntax for a CTE is as follows:

WITH cte_name (column1, column2, ...) AS (
    -- CTE query
    SELECT ...
-- Main query that references the CTE
FROM cte_name;


WITH Clause: The WITH clause introduces the CTE and defines its name and columns (if any). It is followed by the AS keyword.

CTE Name: This is the name you give to your CTE, which can be used to reference the temporary result set later in the query.

Column List: Optionally, you can specify a list of column names in parentheses after the CTE name. This is especially useful when the CTE is expected to return multiple columns.

AS Keyword: This keyword indicates the beginning of the CTE query.

CTE Query: This is a SELECT statement that defines the CTE. It can include joins, aggregations, and other SQL operations.


Let’s consider a simple example where you want to retrieve employees along with their managers from an employee table:

WITH EmployeeCTE AS (

    e.EmployeeName AS Employee,
    m.EmployeeName AS Manager
    EmployeeCTE e
    EmployeeCTE m ON e.ManagerID = m.EmployeeID;

In this example, EmployeeCTE is the CTE that selects relevant columns from the Employee table. The main query then uses this CTE to perform a self-join and retrieve employees along with their managers.

Benefits of CTEs

Readability: CTEs enhance the readability of complex queries by breaking them into modular, named components.

Code Reusability: Since CTEs can be referenced multiple times in a query, they promote code reusability and reduce redundancy.

Recursive Queries: CTEs are often used for recursive queries, where a query refers to its own output. This is especially useful for hierarchical data structures like organizational charts.

In summary, Common Table Expressions are a powerful feature in SQL that contribute to code organization, readability, and reusability. They are particularly handy when dealing with complex queries and recursive relationships in database tables.