SQL CTEs Explained: Using Common Table Expressions in MySQL, PostgreSQL & SQL Server
Using Common Table Expressions (CTEs) Across Databases
Write a SQL query using a Common Table Expression (CTE) to calculate department-wise averages, ensuring compatibility across MySQL, PostgreSQL, and SQL Server.
Solution:
-- Compatible across MySQL, PostgreSQL, and SQL Server
WITH DepartmentAverages AS (
SELECT DepartmentID, AVG(Salary) AS AvgSalary
FROM Employees
GROUP BY DepartmentID
)
SELECT e.EmployeeID, e.Name, e.DepartmentID, d.AvgSalary
FROM Employees e
JOIN DepartmentAverages d ON e.DepartmentID = d.DepartmentID;
Explanation:
- Purpose of the Query :
- The goal is to demonstrate how CTEs work consistently across database systems.
- Key Components :
- WITH DepartmentAverages: Defines a CTE for department-wise averages.
- JOIN: Links employees to their department averages.
- Why Use CTEs?:
- CTEs simplify complex queries and improve readability.
- They are widely supported across platforms.
- Real-World Application :
- In reporting systems, CTEs simplify calculations and aggregations.
Additional Notes:
- Use CTEs for reusable subqueries and modular logic.
- Test queries on all target platforms to ensure compatibility.
- Important Considerations:
- Avoid nesting too many CTEs for performance reasons.
For more Practice: Solve these Related Problems:
- Write a SQL query using a CTE to calculate the total sales for each product, ensuring compatibility across MySQL, PostgreSQL, and SQL Server.
- Write a SQL query using a CTE to find the average score for each student, ensuring compatibility across MySQL, PostgreSQL, and SQL Server.
- Write a SQL query using a CTE to calculate the total budget for each department, ensuring compatibility across MySQL, PostgreSQL, and SQL Server.
- Write a SQL query using a CTE to find the highest-paid employee in each department, ensuring compatibility across MySQL, PostgreSQL, and SQL Server.
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Previous SQL Exercise:Writing a Query that uses CASE Statements Across Databases.
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