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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