An Overview of SQL Common Table Expressions (CTE)

A Common Table Expression (CTE) in SQL is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. CTEs make complex queries more readable and manageable by breaking them into modular, named components. Here are some key points about Common Table Expressions:

1. Syntax:

• The basic syntax for a CTE is as follows:

WITH cte_name (column1, column2, ...) AS (
    -- CTE query definition here
    SELECT ...
)
-- SQL statement that references the CTE
SELECT * FROM cte_name;

2. Recursive CTEs:

• CTEs can be recursive, allowing you to perform hierarchical queries. Recursive CTEs have a UNION ALL clause that refers to the CTE itself.

WITH RecursiveCTE (ID, ParentID, Name) AS (
    SELECT ID, ParentID, Name FROM YourTable WHERE ParentID IS NULL
    UNION ALL
    SELECT t.ID, t.ParentID, t.Name FROM YourTable t
    JOIN RecursiveCTE r ON t.ParentID = r.ID
)
SELECT * FROM RecursiveCTE;

3. Multiple CTEs:

• You can define and use multiple CTEs in a single query, and they are evaluated in the order they are defined.

WITH CTE1 AS (
    -- Definition for CTE1
),
CTE2 AS (
    -- Definition for CTE2
)
SELECT * FROM CTE1
JOIN CTE2 ON ...

4. Non-Recursive CTEs:

• Most CTEs are non-recursive, meaning they don’t…