相關子查詢

相關（也稱為同步或協調）子查詢是巢狀查詢，它們引用其外部查詢的當前行：

SELECT EmployeeId
    FROM Employee AS eOuter
    WHERE Salary > (
       SELECT AVG(Salary)
       FROM Employee eInner
       WHERE eInner.DepartmentId = eOuter.DepartmentId
    )

子查詢 SELECT AVG(Salary) ... 是*相關的，*因為它從其外部查詢引用 Employee row eOuter。