Q.Using the Employee table, write an SQL query to display each department along with the number of employees in it and the average salary of the department.

This needs a per-department summary, which is exactly what GROUP BY produces. …

When a summary is wanted for each department separately, we use the GROUP BY clause to split the rows into groups sharing the same Dept, and then apply the aggregate functions to each group.

SELECT Dept, COUNT(*) AS NumEmployees, AVG(Salary) AS AvgSalary
FROM Employee
GROUP BY Dept;

The result has one row per department:

To also sort the output, add ORDER BY — for example ORDER BY AvgSalary DESC to list …

Do not select a column such as Name alongside a GROUP BY Dept summary — each department has many names but one count, so it is invalid. Every selected col …