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)
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
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;
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
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
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