12. SQL -- 查询解析步骤

标签：customers；sql查询；分析步骤

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 和 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

略

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

略

SELECT * FROM STEP8
 

--Step9 ，应用 DISTINCT 语句，此处无此需求

略

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

略

SELECT * FROM STEP11

本文出自 “Ricky's Blog” 博客，请务必保留此出处http://57388.blog.51cto.com/47388/1622857

12. SQL -- 查询解析步骤

标签：customers；sql查询；分析步骤

原文地址：http://57388.blog.51cto.com/47388/1622857