Using Common Table Expressions (CTEs)

1. Introduction

Common Table Expressions (CTEs) are a powerful feature in SQL that allows you to define temporary result sets that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. CTEs enhance readability and organization of complex queries.

2. What Are CTEs?

A CTE is defined using the WITH clause followed by a query that returns a result set. CTEs can be recursive or non-recursive, allowing for complex hierarchical queries or simpler data manipulations.

3. Syntax

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

4. Examples

4.1 Basic Example

WITH EmployeeCTE AS (
    SELECT EmployeeID, FirstName, LastName
    FROM Employees
    WHERE Department = 'Sales'
)
SELECT * FROM EmployeeCTE;

4.2 Recursive CTE Example

WITH RecursiveCTE AS (
    SELECT EmployeeID, ManagerID, FirstName, LastName
    FROM Employees
    WHERE ManagerID IS NULL
    UNION ALL
    SELECT e.EmployeeID, e.ManagerID, e.FirstName, e.LastName
    FROM Employees e
    INNER JOIN RecursiveCTE r ON e.ManagerID = r.EmployeeID
)
SELECT * FROM RecursiveCTE;

5. Best Practices

• Use CTEs to simplify complex queries.
• Limit the scope of CTEs to avoid performance issues.
• Prefer CTEs over subqueries for better readability.
• Test performance; sometimes using indexed views may be more efficient.

6. FAQ