关于SQLServer2005的学习笔记——子查询
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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','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
让我们继续看看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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