ASIF

A Common Table Expression (CTE) is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement in SQL Server. CTEs are useful for simplifying complex queries and improving code readability by breaking down a query into smaller, more manageable parts. CTEs are defined using the WITH statement and can be recursive or non-recursive.

Here’s the basic syntax for defining a non-recursive CTE:

WITH CTE_Name (Column1, Column2, ...) AS (
-- Subquery or SQL statement that defines the CTE
)
-- Query that references the CTE

And here’s the syntax for defining a recursive CTE:

WITH CTE_Name (Column1, Column2, ...) AS (
-- Anchor member: Initial query
SELECT ...
UNION ALL
-- Recursive member: Subquery that references the CTE itself
SELECT ...
)
-- Query that references the CTE

Here are some key points about CTEs in SQL Server:

▶Non-Recursive CTE: Non-recursive CTEs are used for simple queries that don’t involve recursion. They are often employed to simplify the query and improve readability. For example:

WITH EmployeesWithSalaryOver50k AS (
SELECT FirstName, LastName, Salary
FROM Employee
WHERE Salary > 50000
)
SELECT * FROM EmployeesWithSalaryOver50k;

WITH EmployeesWithSalaryOver50k AS (
SELECT FirstName, LastName, Salary
FROM Employee
WHERE Salary > 50000
)
SELECT * FROM EmployeesWithSalaryOver50k;

▶Recursive CTE: Recursive CTEs are used for queries involving hierarchical or recursive data structures, such as organizational hierarchies or bill of materials. They include both an anchor member (the initial query) and a recursive member (subquery that references the CTE itself) connected by a UNION ALL operation.

WITH EmployeeHierarchy (EmployeeID, ManagerID, EmployeeName, Depth) AS (
SELECT EmployeeID, ManagerID, EmployeeName, 0
FROM Employees
WHERE ManagerID IS NULL -- Anchor member

UNION ALL

SELECT E.EmployeeID, E.ManagerID, E.EmployeeName, EH.Depth + 1
FROM Employees E
INNER JOIN EmployeeHierarchy EH ON E.ManagerID = EH.EmployeeID -- Recursive member
)
SELECT EmployeeID, ManagerID, EmployeeName, Depth
FROM EmployeeHierarchy;

▶CTEs Improve Readability: CTEs are often used to break down complex queries into smaller, more manageable parts, improving code readability and maintainability. They also help avoid duplicating the same subquery logic in multiple places within a larger query.

▶Scope: CTEs are only valid within the scope of the query in which they are defined. They cannot be referenced in other queries, and they do not persist beyond the query execution.

▶ Performance: CTEs can be optimized by the SQL Server query optimizer, and they are usually as efficient as equivalent subqueries or derived tables. However, performance may vary depending on the specific query and indexing.

Common Table Expressions are a powerful tool for simplifying complex SQL queries and handling recursive data structures.