SQL CTE Examples A Common Table Expression (CTE) is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. Here are a few examples of how to use CTEs in SQL:
- Recursive SQL CTE Examples
A recursive CTE is used to traverse hierarchical data such as an organizational chart or a tree structure. Here is an example of a recursive CTE that generates a list of all employees and their managers in an organization:
WITH EmployeeHierarchy AS ( SELECT EmployeeID, Name, ManagerID FROM Employees WHERE ManagerID IS NULL UNION ALL SELECT e.EmployeeID, e.Name, e.ManagerID FROM Employees e JOIN EmployeeHierarchy eh ON e.ManagerID = eh.EmployeeID ) SELECT * FROM EmployeeHierarchy;
- Non-recursive SQL CTE Examples
A non-recursive CTE is used to simplify complex queries by breaking them down into smaller, more manageable parts. Here is an example of a non-recursive CTE that calculates the average price of all products in a certain category:
WITH CategoryProducts AS ( SELECT ProductID, ProductName, CategoryID, Price FROM Products WHERE CategoryID = 1 ) SELECT AVG(Price) AS AveragePrice FROM CategoryProducts;
- CTE with multiple queries
You can use a CTE to simplify queries that involve multiple subqueries. Here is an example of a CTE that calculates the total sales for each employee in a certain department:
WITH EmployeeSales AS ( SELECT EmployeeID, SUM(SalesAmount) AS TotalSales FROM Sales WHERE DepartmentID = 2 GROUP BY EmployeeID ) SELECT e.EmployeeName, es.TotalSales FROM Employees e JOIN EmployeeSales es ON e.EmployeeID = es.EmployeeID;
These are just a few examples of how you can use CTEs in SQL to simplify complex queries and improve performance.
Common Table Expression (CTEs) are a powerful tool for creating complex queries that are easier to read and maintain. They can be used for a wide range of tasks, from retrieving hierarchical data to filtering and ranking result sets. By mastering the use of CTEs, SQL developers can create more efficient and effective queries that provide better insights into their data.
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