你真的会玩SQL吗？你所不知道的 数据聚合

CREATE TABLE EmpOrders
    (
      empid INT NOT NULL ,
      ordmonth DATE NOT NULL ,--只取到月份2015-07-07
      qty INT NOT NULL ,
      PRIMARY KEY ( empid, ordmonth )
    )
go 
INSERT  INTO EmpOrders
        ( empid ,
          ordmonth ,
          qty 
        )
        SELECT  o.empid ,
                DATEADD(MONTH, DATEDIFF(MONTH, 0, o.orderdate), 0) AS ordmonth ,
                SUM(qty) AS qty
        FROM    Sales.Orders AS o
                JOIN Sales.OrderDetails AS od ON o.orderid = od.orderid
        GROUP BY empid ,
                DATEADD(MONTH, DATEDIFF(MONTH, 0, o.orderdate), 0)

SELECT  empid ,
        ordmonth ,
        qty
FROM    EmpOrders
ORDER BY empid ,
        ordmonth

SELECT  o1.empid ,
        o1.ordmonth ,
        o1.qty AS qtythismonth ,
        SUM(o2.qty) AS totalqty ,
        CAST(AVG(1. * o2.qty) AS NUMERIC(12, 2)) AS avgqty
FROM    EmpOrders AS o1
        JOIN EmpOrders AS o2 ON o2.empid = o1.empid
                                AND o2.ordmonth <= o1.ordmonth
GROUP BY o1.empid ,
        o1.ordmonth ,
        o1.qty
ORDER BY o1.empid ,
        o1.ordmonth

SELECT  o1.empid ,
        o1.ordmonth ,
        o1.qty AS qtythismonth ,
        SUM(o2.qty) AS totalqty ,
        CAST(AVG(1. * o2.qty) AS NUMERIC(12, 2)) AS avgqty
FROM    EmpOrders AS o1
        JOIN EmpOrders AS o2 ON o2.empid = o1.empid
                                AND o2.ordmonth <= o1.ordmonth
GROUP BY o1.empid ,
        o1.ordmonth ,
        o1.qty
        HAVING SUM(o2.qty)<1000
ORDER BY o1.empid ,
        o1.ordmonth