关于SQLServer2005的学习笔记——子查询
2010-01-11 13:21
459 查看
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
让我们继续看看
SQL Server
是如何处理的
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','WBQ',1,TO_DATE('2000-01-21','YYYY-MM-DD')); INSERT INTO Orders VALUES('110003','WBQ',2,TO_DATE('2000-01-11','YYYY-MM-DD')); INSERT INTO Orders VALUES('110004','WBQ',3,TO_DATE('2000-02-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110005','CZH',1,TO_DATE('2000-02-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110006','CZH',2,TO_DATE('2000-03-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110007','CZH',2,TO_DATE('2000-03-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110008','KIDD',3,TO_DATE('2000-02-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110009','KIDD',1,TO_DATE('2000-04-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110010','KIDD',2,TO_DATE('2000-03-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110011','KIDD',3,TO_DATE('2000-02-01','YYYY-MM-DD')); INSERT INTO Orders VALUES('110012','CZH',3,TO_DATE('2000-01-10','YYYY-MM-DD')); COMMIT; SELECT EmployeeID,OrderDate,OrderID,CustomerID FROM Orders ORDER BY EmployeeID,OrderDate,OrderID |
-- 层递直至实现唯一为止 SELECT EmployeeID,OrderDate,OrderID,CustomerID FROM Orders WHERE (EmployeeID,OrderDate,OrderID) IN (SELECT EmployeeID,OrderDate,MAX(OrderID) FROM Orders WHERE (EmployeeID,OrderDate) IN (SELECT EmployeeID,Max(OrderDate) FROM Orders GROUP BY EmployeeID) GROUP BY EmployeeID,OrderDate) ORDER BY EmployeeID,OrderDate,OrderID,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,OrderID,CustomerID FROM Orders WHERE (EmployeeID,OrderDate) IN (SELECT EmployeeID,Max(OrderDate) FROM Orders GROUP BY EmployeeID) ORDER BY EmployeeID,OrderDate,OrderID,CustomerID |
--Error ,这是个错误的表达式 SELECT EmployeeID,OrderDate,OrderID,CustomerID FROM Orders WHERE (EmployeeID,OrderDate,OrderID) IN (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,OrderDate,OrderID,CustomerID FROM ( SELECT EmployeeID,OrderDate,OrderID,CustomerID, RANK() OVER (PARTITION BY EmployeeID ORDER BY OrderDate DESC,OrderID DESC) Rank FROM Orders ) a WHERE a.Rank=1 |
SQL Server
是如何处理的
CREATE TABLE Orders ( OrderID VARCHAR(6), CustomerID VARCHAR(6), EmployeeID INT, OrderDate DATETIME ); INSERT INTO Orders VALUES('110001','WBQ',1,'2000-01-11'); INSERT INTO Orders VALUES('110002','WBQ',1,'2000-01-21'); INSERT INTO Orders VALUES('110003','WBQ',2,'2000-01-11'); INSERT INTO Orders VALUES('110004','WBQ',3,'2000-02-01'); INSERT INTO Orders VALUES('110005','CZH',1,'2000-02-01'); INSERT INTO Orders VALUES('110006','CZH',2,'2000-03-01'); INSERT INTO Orders VALUES('110007','CZH',2,'2000-03-01'); INSERT INTO Orders VALUES('110008','KIDD',3,'2000-02-01'); INSERT INTO Orders VALUES('110009','KIDD',1,'2000-04-01'); INSERT INTO Orders VALUES('110010','KIDD',2,'2000-03-01'); INSERT INTO Orders VALUES('110011','KIDD',3,'2000-02-01'); INSERT INTO Orders VALUES('110012','CZH',3,'2000-01-10'); COMMIT; |
-- 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,OrderID,CustomerID FROM Orders O1 WHERE rderDate= (SELECT MAX(OrderDate) FROM Orders O2 WHERE O1.EmployeeID=O2.EmployeeID) ORDER BY EmployeeID,OrderDate,OrderID,CustomerID 等效于 Oracle 的以下语句 SELECT EmployeeID,OrderDate,OrderID,CustomerID FROM Orders WHERE (EmployeeID,OrderDate) IN (SELECT EmployeeID,Max(OrderDate) FROM Orders GROUP BY EmployeeID) ORDER BY EmployeeID,OrderDate,OrderID,CustomerID |
正确的 SQLServer 子查询写法,用两个 MAX 求得唯一值 SELECT EmployeeID,OrderDate,OrderID,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,OrderDate,OrderID,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,1,8) AS DATETIME) AS OrderDate, CAST(SUBSTRING(BinStr,9,6) AS VARCHAR) AS OrderID, CAST(SUBSTRING(BinStr,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,OrderDate,OrderID,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,OrderDate,OrderID,CustomerID |
-- 本例使用了 IN 子查询,可以自定义返回的 TOP N 条数 SELECT EmployeeID,OrderDate,OrderID,CustomerID FROM Orders O1 WHERE OrderID IN (SELECT TOP(1) OrderID FROM Orders O2 WHERE O1.EmployeeID=O2.EmployeeID ORDER BY OrderDate DESC,OrderID DESC,CustomerID ) ORDER BY EmployeeID,OrderDate,OrderID,CustomerID |
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