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关于SQLServer2005的学习笔记——SQL查询解析步骤

 
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最近一来因工作上的事情比较闹心,没心事再研究 SQLServer2005 的体系结构;一来关于体系结构确实过于复杂,远远不如应用来的直接明了,所以暂时搁笔。

出于工作上的需要,对应用开发需要更多的了解,所以把心事暂时放到这方面。

先从最简单的 SQL 入手来分析一下 SQL 的执行步骤,为什么了解执行步骤,其实与 JOIN 后的 ON 条件和 WHERE 条件容易混淆有关系。

是先执行 ON 还是先执行 WHERE ,很大程度上会决定 SQL 的结果集正确与否。

CREATE TABLE Customers

(

CustomerID CHAR(5) NOT NULL PRIMARY KEY,

City VARCHAR(10) NOT NULL

);

CREATE TABLE Orders

(

OrderID INT NOT NULL PRIMARY KEY,

CustomerID CHAR(5) NULL REFERENCES Customers(CustomerID)

);

INSERT INTO Customers VALUES('FISSA','Madrid');

INSERT INTO Customers VALUES('FRNDO','Madrid');

INSERT INTO Customers VALUES('KRLOS','Madrid');

INSERT INTO Customers VALUES('MRPHS','Zion');

INSERT INTO Orders VALUES(1,'FRNDO');

INSERT INTO Orders VALUES(2,'FRNDO');

INSERT INTO Orders VALUES(3,'KRLOS');

INSERT INTO Orders VALUES(4,'KRLOS');

INSERT INTO Orders VALUES(5,'KRLOS');

INSERT INTO Orders VALUES(6,'MRPHS');

INSERT INTO Orders VALUES(7,NULL);

试看看以上两个语句有什么不同,你就会发现很有趣的现象。

SELECT C.CustomerID,COUNT(O.OrderID) AS NumOrders

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

WHERE C.City='Madrid'

GROUP BY C.CustomerID

HAVING COUNT(O.OrderID)<3

ORDER BY NumOrders;

SELECT C.CustomerID,COUNT(O.OrderID) AS NumOrders

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

AND C.City='Madrid'

GROUP BY C.CustomerID

HAVING COUNT(O.OrderID)<3

ORDER BY NumOrders;

--Step1 ,首先对 FROM 后面的表进行笛卡尔乘积,生成虚表 STEP1

WITH STEP1

AS

(

SELECT C.CustomerID C_CustomerID,C.City C_City,O.OrderID O_OrderID,O.CustomerID O_CustomerID

FROM Customers C,Orders O

)

SELECT * FROM STEP1

--Step2 ,再次应用 ON 语句中的条件,如果没有外关联的话,这里的 ON WHERE 实际上是没有什么差别的,生成虚表 STEP2

WITH STEP2

AS

(

SELECT C.CustomerID C_CustomerID,C.City C_City,O.OrderID O_OrderID,O.CustomerID O_CustomerID

FROM Customers C

JOIN Orders O

ON C.CustomerID=O.CustomerID

)

SELECT * FROM STEP2

--Step3 ,如果指定了 OUTER JOIN SQL 会自动把 STEP2 表中未匹配的行作为外部行添加到 STEP3 中,此处找到了 CustomerID=FISSA,City=Madrid ,这个没有订单但又有相关名字的用户

WITH STEP3

AS

(

SELECT C.CustomerID C_CustomerID,C.City C_City,O.OrderID O_OrderID,O.CustomerID O_CustomerID

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

)

SELECT * FROM STEP3

--Step4 ,应用 WHERE 条件,过滤不符合条件的记录

AS

(

SELECT C.CustomerID C_CustomerID,C.City C_City,O.OrderID O_OrderID,O.CustomerID O_CustomerID

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

WHERE C.City='Madrid'

)

SELECT * FROM STEP4

--Step5 ,对以上的结果集进行分组

WITH STEP5

AS

(

SELECT C.CustomerID,COUNT(O.OrderID) AS NumOrders

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

WHERE C.City='Madrid'

GROUP BY C.CustomerID

)

SELECT * FROM STEP5

--Step6 ,处理 CUBE ROLLUP 之类的语句,此处无此需求

SELECT * FROM STEP6

--Step7 处理 Having 筛选器,与 WHERE 条件有些类似

WITH STEP7

AS

(

SELECT C.CustomerID,COUNT(O.OrderID) AS NumOrders

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

WHERE C.City='Madrid'

GROUP BY C.CustomerID

HAVING COUNT(O.OrderID)<3

)

SELECT * FROM STEP7

--Step8 ,处理 SELECT 列表,即别名转换把 COUNT(O.OrderID) 转换成 NumOrders

SELECT * FROM STEP8

--Step9 ,应用 DISTINCT 语句,此处无此需求

SELECT * FROM STEP9

--Step10 ,应用 ORDER BY 语句进行排序

SELECT C.CustomerID,COUNT(O.OrderID) AS NumOrders

FROM Customers C

LEFT OUTER JOIN Orders O

ON C.CustomerID=O.CustomerID

WHERE C.City='Madrid'

GROUP BY C.CustomerID

HAVING COUNT(O.OrderID)<3

ORDER BY NumOrders

--Step11 ,执行 TOP 选项,此处无此需求

SELECT * FROM STEP11

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