关于SQLServer2005的学习笔记——子查询

SQL Server 的子查询给人的感觉一向不是很好用， IN 子查询无法实现多列的子查询，很多情况下又需要进行自我的子查询操作，比如取员工的最新订单之类的问题。

以下 sql 和案例来之于 <sqlServer2005 技术内幕 T-sql 查询 > 一书，不过适当的做了些编排和自己的理解。

 

让我们先来看看 Oracle 是怎么处理子查询的

CREATE TABLE Orders (   OrderID     VARCHAR2(6),   CustomerID  VARCHAR2(6),   EmployeeID  INT,   OrderDate   DATE ); TruncATE TABLE Orders; INSERT INTO Orders VALUES('110001','WBQ',1,TO_DATE('2000-01-11','YYYY-MM-DD')); INSERT INTO Orders VALUES('110002',TO_DATE('2000-01-21','YYYY-MM-DD')); INSERT INTO Orders VALUES('110003',2,'YYYY-MM-DD')); INSERT INTO Orders VALUES('110004',3,TO_DATE('2000-02-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110005','CZH','YYYY-MM-DD')); INSERT INTO Orders VALUES('110006',TO_DATE('2000-03-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110007','YYYY-MM-DD')); INSERT INTO Orders VALUES('110008','KIDD','YYYY-MM-DD')); INSERT INTO Orders VALUES('110009',TO_DATE('2000-04-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110010','YYYY-MM-DD')); INSERT INTO Orders VALUES('110011','YYYY-MM-DD')); INSERT INTO Orders VALUES('110012',TO_DATE('2000-01-10','YYYY-MM-DD')); COMMIT; SELECT EmployeeID,OrderDate,OrderID,CustomerID   FROM Orders   ORDER BY EmployeeID,OrderID

 

-- 层递直至实现唯一为止 SELECT EmployeeID,CustomerID   FROM Orders   WHERE (EmployeeID,OrderID) IN        (SELECT EmployeeID,MAX(OrderID)           FROM Orders          WHERE (EmployeeID,OrderDate) IN                (SELECT EmployeeID,Max(OrderDate)                   FROM Orders                  GROUP BY EmployeeID)          GROUP BY EmployeeID,OrderDate)   ORDER BY EmployeeID,CustomerID

 

-- 多值，通常情况下该语句即可，不过在本例中 EmployeeID,OrderDate 并不唯一，导致结果非所需的 SELECT EmployeeID,OrderDate) IN        (SELECT EmployeeID,Max(OrderDate)           FROM Orders          GROUP BY EmployeeID)   ORDER BY EmployeeID,CustomerID

 

--Error ，这是个错误的表达式 SELECT EmployeeID,MAX(OrderDate),MAX(OrderID)           FROM Orders          GROUP BY EmployeeID)

 

-- 使用分析函数，也可以实现相应的子查询        SELECT EmployeeID,CustomerID FROM (   SELECT EmployeeID,CustomerID,          RANK() OVER (PARTITION BY EmployeeID ORDER BY OrderDate DESC,OrderID DESC) Rank     FROM Orders ) a   WHERE a.Rank=1

 

让我们继续看看 sql Server 是如何处理的

CREATE TABLE Orders (   OrderID     VARCHAR(6),   CustomerID  VARCHAR(6),   OrderDate   DATETIME ); INSERT INTO Orders VALUES('110001','2000-01-11'); INSERT INTO Orders VALUES('110002','2000-01-21'); INSERT INTO Orders VALUES('110003','2000-01-11'); INSERT INTO Orders VALUES('110004','2000-02-01'); INSERT INTO Orders VALUES('110005','2000-02-01'); INSERT INTO Orders VALUES('110006','2000-03-01'); INSERT INTO Orders VALUES('110007','2000-03-01'); INSERT INTO Orders VALUES('110008','2000-02-01'); INSERT INTO Orders VALUES('110009','2000-04-01'); INSERT INTO Orders VALUES('110010','2000-03-01'); INSERT INTO Orders VALUES('110011','2000-02-01'); INSERT INTO Orders VALUES('110012','2000-01-10'); COMMIT;

 

-- 多值，通常情况下该语句即可，不过在本例中 EmployeeID,CustomerID   FROM Orders O1   WHERE rderDate=        (SELECT MAX(OrderDate)           FROM Orders O2          WHERE O1.EmployeeID=O2.EmployeeID)   ORDER BY EmployeeID,CustomerID 等效于 Oracle 的以下语句 SELECT EmployeeID,CustomerID

 

正确的 sqlServer 子查询写法，用两个 MAX 求得唯一值 SELECT EmployeeID,CustomerID   FROM Orders O1   WHERE rderDate=        (SELECT MAX(OrderDate)           FROM Orders O2          WHERE O1.EmployeeID=O2.EmployeeID)    AND rderID=         (SELECT Max(OrderID)           FROM Orders O2          WHERE O1.EmployeeID=O2.EmployeeID            AND O1.OrderDate=O2.OrderDate)   ORDER BY EmployeeID,CustomerID

 

-- 本例使用一种变通的方法，把几个应有的附加条件加进来然后返回，比较难以理解 SELECT   CAST(SUBSTRING(BinStr,8) AS DATETIME) AS OrderDate,   CAST(SUBSTRING(BinStr,9,6) AS VARCHAR) AS OrderID,15,6) AS VARCHAR) AS CustomerID FROM (SELECT EmployeeID,              MAX(CAST(OrderDate AS BINARY(8))                 +CAST(OrderID AS BINARY(6))                 +CAST(CustomerID AS BINARY(6))) AS BinStr         FROM Orders        GROUP BY EmployeeID) D;

 

-- 本例中在子查询中使用 TOP+Order 排序的方式获取相应的第一行值 SELECT EmployeeID,CustomerID   FROM Orders O1   WHERE rderID=   (SELECT TOP(1) OrderID      FROM Orders O2     WHERE O1.EmployeeID=O2.EmployeeID     ORDER BY OrderDate DESC,OrderID DESC,CustomerID    )   ORDER BY EmployeeID,CustomerID

 

-- 本例使用了 IN 子查询，可以自定义返回的 TOP N 条数 SELECT EmployeeID,CustomerID   FROM Orders O1   WHERE OrderID IN   (SELECT TOP(1) OrderID      FROM Orders O2     WHERE O1.EmployeeID=O2.EmployeeID     ORDER BY OrderDate DESC,CustomerID    ) ORDER BY EmployeeID,CustomerID