GROUP BYとの組み合わせ。普通に書けば良いです。 SELECT d.DepartmentName AS 部門名 , AVG(s.Amount) AS 部門別平均給与額 FROM Salary AS s JOIN BelongTo AS b ON s.EmployeeID = b.EmployeeID JOIN Departments AS d ON b.DepartmentID = d.DepartmentI…