SELECTE.EmployeeID, E.EmployeeName, M.EmployeeName AS Manager FROM Employee_T E, Employee_T M WHERE E.EmployeeSupervisor = M.EmployeeID; [self join, find out all the managing relationships,]SELECTCOUNT(OrderID) as N_APP FROM OrderLine_T WHERE ProductID = 4 AND OrderID IN (SELECTOrderID FROM Order…

所有不是通过索引直接返回排序结果的排序都叫 FileSort 排序。FileSort 并不代表通过磁盘文件进行排序，而只是说进行了一个排序操作，至于排序操作是否使用了磁盘文件或临时表取决于 MySQL 服务器对排序参数的设置和需要排序数据的大小。

(CREATE TABLE Order_T ( OrderID NUMBER(5,0) NOT NULL, OrderDate DATE DEFAULT SYSDATE,CustomerID NUMBER(3,0), CONSTRAINT Order_PK PRIMARY KEY (OrderID)); (Default DATE format: DD-MON-YY HH:MI:SS AM/PM(DROP TABLE Order_T; DROP TABLE Customer-T CASCADE CONSTRAINTS;(INSERT INTO Order_T VALUES (12001,…

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