SQL練習--某書第四章 表表達式

第一種：派生表

 （就是子查詢感覺變成一個表）從這個表from或者join

但是需要特別注意，派生表的orderby一點作用都沒有，不能用！！！

第二特別注意：

使用派生表的一個最常見情景就是，selelct子句處理之前的查詢語句中無法引用select子句分配的列別名，因爲selelct執行在他們之後

情景3：多個嵌套引用

---------------------------爲了表達方便，可以用with+表名+As的形式

 

 

# 1-1.
SELECT empid,Max(orderdate) AS maxorderdate
  FROM Sales.Orders
 GROUP BY empid;
  
#1-2.
             
SELECT so2.empid,so2.orderdate,so2.orderid,so2.custid
  FROM Sales.Orders AS so2
  JOIN （
        SELECT empid,Max(orderdate) AS maxorderdate
          FROM Sales.Orders
		 GROUP BY empid）AS so1
	ON so2.empid = so1.empid
   AND so2.orderdate = so1.maxorderdate;

-- 特別注意：爲什麼第二題不像第一題那樣直接一個呢？alter
-- 因爲groupby的特性，你在select裏面不能出現多於的列

#2-1
SELECT orderid,orderdate,custid,empid,
       ROW_NUMBER() OVER(ORDER BY orderdate,orderid) AS rownum
  FROM Sales.Orders;
  
#2-2
SELECT so.orderid,so.orderdate,so.custid,so.empid,so.rownum
  FROM (
       SELECT orderid,orderdate,custid,empid,
              ROW_NUMBER() OVER(ORDER BY orderdate,orderid) AS rownum
         FROM Sales.Orders
        ) AS so
  WHERE so.rownum BETWEEN 11 AND 20;
  
-- 特別注意，爲什麼我不這樣呢
SELECT orderid,orderdate,custid,empid,
       ROW_NUMBER() OVER(ORDER BY orderdate,orderid) AS rownum
  FROM Sales.Orders
 WHERE so.rownum BETWEEN 11 AND 20;
-- 因爲select執行順序在where之後，所有where裏面的rownum無法執行

#3.遞歸 
WITH EmpsCTE AS
(
SELECT empid,mgrid,firstname,lastname
  FROM HR.Employees
 WHERE empid = 9
 UNION ALL
SELECT P.empid,P.mgrid,P.firstname,P.lastname
  FROM EmpsCTE AS C
  JOIN HR.Employees AS P
    ON C.mgrid = P.empid
    )
SELECT  empid,mgrid,firstname,lastname
  FROM EmpsCTE
#4.1
SELECT so.empid,YEAR(sod.orderdate),SUM(sod.qty) AS qty
  FROM Sales.Orders
  JOIN Sales.OrderDetails
    ON so.orderid = sod.orderid
 GROUP BY empid,YEAR(sod.orderdate);
 
 #4.2
SELECT sv.empid,sv.orderyear,sv.qty,
      SELECT SUM(qty）
		FROM Sales.VempOrders AS sv2
	   WHERE sv1.empid = sv2.empid
         AND sv2.orderyear <= sv1.orderyear)AS runqty
   FROM Sales.VemOrders AS sv
  ORDER BY sv1.empid,sv1.orderyear;
 GROUP BY empid,YEAR(sv.orderdate)

重點1-2，2-2，3-2，4-2