A Common Table Expression (CTE) in SQL Server is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. It simplifies complex queries and improves readability. CTEs are defined using the WITH keyword.

Here' a basic example of a CTE:

WITH CTE_Name AS (
    SELECT column1, column2
    FROM mytable
    WHERE condition
)
SELECT column1, column2
FROM CTE_Name;

Types of CTEs

1. Simple CTE: Used for straightforward querying.

   sql

   WITH EmployeeCTE AS (
       SELECT EmployeeID, FirstName, LastName
       FROM Employees
   )
   SELECT EmployeeID, FirstName, LastName
   FROM EmployeeCTE;
   

2. Recursive CTE: Used for hierarchical or recursive data structures, such as organizational charts or tree structures.

   sql

   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 EmployeeID, ManagerID, FirstName, LastName
   FROM RecursiveCTE;
   

Benefits of Using CTEs

• Improved Readability: Breaks down complex queries into simpler, more readable parts.

• Reusability: You can reference the CTE multiple times in the main query.

• Modularity: Makes maintaining the code easier by segmenting parts of the query.

Would you like to explore a more specific use case for CTEs or see an example involving more complex logic?