A Common Table Expression, is a TEMPORARY named result set, that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement.
CTE stands for Common Table Expression. In SQL Server, a Common Table Expression is a temporary result set that can be referenced within the context of a single SELECT, INSERT, UPDATE, or DELETE statement. CTEs provide a way to simplify complex queries, improve readability, and reuse the intermediate result set within the same query.
CTEs are defined using the WITH clause and are typically used in conjunction with the SELECT statement. The result set of the CTE is available only for the duration of the query execution, and it is not stored permanently in the database.
The syntax of a CTE in SQL Server is as follows:
WITH CTE_Name (column1, column2, ..., columnN) AS (
-- CTE query definition here
-- It can be a SELECT, INSERT, UPDATE, or DELETE statement
)
-- Main query using the CTE
SELECT column1, column2, ..., columnN
FROM CTE_Name
WHERE <condition>;
Here's a simple example to illustrate how a CTE works:
Consider a table named "Employees" with columns "EmployeeID," "FirstName," and "Salary." We want to find employees who have a salary greater than a certain threshold and display their information along with their salary rank. We can use a CTE to achieve this:
WITH RankedEmployees AS (
SELECT EmployeeID, FirstName, Salary,
RANK() OVER (ORDER BY Salary DESC) AS SalaryRank
FROM Employees
)
SELECT EmployeeID, FirstName, Salary, SalaryRank
FROM RankedEmployees
WHERE Salary > 50000;
In the above query, we first define the CTE named "RankedEmployees." The CTE uses the RANK() window function to calculate the rank of employees based on their salary in descending order. Then, in the main query, we retrieve data from the CTE and filter the results to show only those employees with a salary greater than 50000.
CTEs are particularly useful when dealing with complex queries that involve multiple subqueries or require the same intermediate result set to be referenced multiple times within the same query. They help in simplifying the query structure and improving the query's maintainability and readability.
