SQLServer 获得用户最新或前n条订单的几种SQL语句小结
场景:有一张用户表,一个订单表,要求获得一个用户对应的最新的一条订单信息。
实现以上要求,我们可以用以下几种方式,但是效率却相差很远。首先我们在Order表中,创建一个索引:
CREATE UNIQUE INDEX idx_eid_odD_oidD ON Orders(EmployeeID,OrderDate DESC,OrderID DESC)
多个OrderId是为了在OrderData相同的情况下,按订单号倒序,是个辅助属性。
方法1:
代码如下:
SELECT EmployeeID,OrderID FROM Orders AS O1
WHERE OrderID = (
SELECT TOP(1)OrderID FROM Orders AS O2
WHERE O1.EmployeeID = O2.EmployeeID
ORDER BY OrderDate DESC ,OrderID DESC
)
如果想获得前n条订单信息,把 = 号改成IN,然后TOP(n)就可以了。
不论是取一条还是多条,即使有索引,数据多的情况下,也是最慢的。
方法2:
代码如下:
SELECT O.EmployeeID,O.OrderID FROM (
SELECT EmployeeID,(SELECT TOP(1)OrderID FROM Orders AS O2 WHERE E.EmployeeID = O2.EmployeeID ORDER BY OrderDate DESC,OrderID DESC) AS OrderID
FROM Employees AS E
) AS EO
INNER JOIN Orders AS O
ON EO.OrderID = O.OrderID
方法2只能取一条信息,不能取多条信息。
在取一条的情况下,这个要比方法1快多了,因为用户相比订单信息要少很多。
方法3:
代码如下:
SELECT E.EmployeeID,O.OrderID FROM Employees AS E
CROSS APPLY (
SELECT TOP(1)* FROM Orders AS O1 WHERE E.EmployeeID = O1.EmployeeID ORDER BY O1.OrderDate DESC,O1.OrderID DESC
) AS O
这个应用到了SQL Server 2005或更高版本的一些新特性,这个效率要比方法2还好。
如果想取得多条,只需更改TOP(n)即可。
APPLY详解,参见
方法4:
代码如下:
SELECT O1.EmployeeID,O1.OrderID
FROM Orders O1 JOIN (
SELECT ROW_NUMBER() OVER(PARTITION BY EmployeeID ORDER BY OrderDate DESC,OrderID DESC) AS RowNumber,*
FROM Orders AS OT
) AS O2
ON O1.OrderID = O2.OrderID
WHERE O2.RowNumber = 1
这个ROW_NUMBER函数也是在SQL Server 2005后新增的,使用这个和方法3查不多,甚至比3更好,但要注意一点是先按EmployeeID分区,然后再排序。
结合以上方法, 建议用方法3。

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