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上次说到SQL查询语句的逻辑执行过程，现在来用一个实例说明一下逻辑执行的过程。
前提：我们有三个表，分别记住客户信息、订单信息和产品信息
客户信息表：Customer
ID，Name，Adress，PhoneNumber
| ID | Name | Adress | PhoneNumber |
| 1 | CompanyA | No.1 Street | 123456 |
| 2 | CompanyB | No.2 Street | 23453 |
| 3 | CompanyC | No.3 Street | 45321 |
| 4 | CompanyD | No.4 Street | 4567890 |
| 5 | CompanyE | No.5 Street | 123890 |
订单信息表：OrderInfo
ID，Cus_ID，Product_ID，OrderDate，Qua
| ID | Cus_ID | Product_ID | OrderDate | Qua |
| 1 | 2 | 2 | 1/1/2012 | 1000 |
| 2 | 4 | 1 | 5/2/2012 | 500 |
| 3 | 1 | 5 | 9/10/2013 | 3069 |
产品信息表：Product
ID，Name，Price
| ID | Name | Price |
| 1 | ABC | 10 |
| 2 | DEF | 16 |
| 3 | GHI | 18 |
| 4 | JKL | 9 |
| 5 | MNO | 100 |
1 SELECT 2 C.Name AS CustomerName, 3 O.OrderDate AS OrderDate, 4 O.Qua, 5 P.Name AS ProductName, 6 P.Price AS Price 7 FROM Customer C 8 INNER JOIN OrderInfo O 9 ON C.ID = O.Cus_ID 10 INNER JOIN Product P 11 ON O.Product_ID = P.ID
现在我们根据上一篇文章中的逻辑执行顺序分析一下查询的结果。
1) FROM Customer 表作为主表 T1
2) JOIN OrderInfo 表作为表 T2，这时将 T1和T2做笛卡尔乘积。得到的虚拟表如下：
| ID | Name | Adress | PhoneNumber | ID | Cus_ID | Product_ID | OrderDate | Qua |
| 1 | CompanyA | No.1 Street | 123456 | 1 | 2 | 2 | 1/1/2012 | 1000 |
| 2 | CompanyB | No.2 Street | 23453 | 1 | 2 | 2 | 1/1/2012 | 1000 |
| 3 | CompanyC | No.3 Street | 45321 | 1 | 2 | 2 | 1/1/2012 | 1000 |
| 4 | CompanyD | No.4 Street | 4567890 | 1 | 2 | 2 | 1/1/2012 | 1000 |
| 5 | CompanyE | No.5 Street | 123890 | 1 | 2 | 2 | 1/1/2012 | 1000 |
| 1 | CompanyA | No.1 Street | 123456 | 2 | 4 | 1 | 5/2/2012 | 500 |
| 2 | CompanyB | No.2 Street | 23453 | 2 | 4 | 1 | 5/2/2012 | 500 |
| 3 | CompanyC | No.3 Street | 45321 | 2 | 4 | 1 | 5/2/2012 | 500 |
| 4 | CompanyD | No.4 Street | 4567890 | 2 | 4 | 1 | 5/2/2012 | 500 |
| 5 | CompanyE | No.5 Street | 123890 | 2 | 4 | 1 | 5/2/2012 | 500 |
| 1 | CompanyA | No.1 Street | 123456 | 3 | 1 | 5 | 9/10/2013 | 3069 |
| 2 | CompanyB | No.2 Street | 23453 | 3 | 1 | 5 | 9/10/2013 | 3069 |
| 3 | CompanyC | No.3 Street | 45321 | 3 | 1 | 5 | 9/10/2013 | 3069 |
| 4 | CompanyD | No.4 Street | 4567890 | 3 | 1 | 5 | 9/10/2013 | 3069 |
| 5 | CompanyE | No.5 Street | 123890 | 3 | 1 | 5 | 9/10/2013 | 3069 |
执行 ON 后边的条件：C.ID = O.Cus_ID，保留结果为 TRUE 的记录，结果如下：
| ID | Name | Adress | PhoneNumber | ID | Cus_ID | Product_ID | OrderDate | Qua | Result |
| 1 | CompanyA | No.1 Street | 123456 | 1 | 2 | 2 | 1/1/2012 | 1000 | FALSE |
| 2 | CompanyB | No.2 Street | 23453 | 1 | 2 | 2 | 1/1/2012 | 1000 | TRUE |
| 3 | CompanyC | No.3 Street | 45321 | 1 | 2 | 2 | 1/1/2012 | 1000 | FALSE |
| 4 | CompanyD | No.4 Street | 4567890 | 1 | 2 | 2 | 1/1/2012 | 1000 | FALSE |
| 5 | CompanyE | No.5 Street | 123890 | 1 | 2 | 2 | 1/1/2012 | 1000 | FALSE |
| 1 | CompanyA | No.1 Street | 123456 | 2 | 4 | 1 | 5/2/2012 | 500 | FALSE |
| 2 | CompanyB | No.2 Street | 23453 | 2 | 4 | 1 | 5/2/2012 | 500 | FALSE |
| 3 | CompanyC | No.3 Street | 45321 | 2 | 4 | 1 | 5/2/2012 | 500 | FALSE |
| 4 | CompanyD | No.4 Street | 4567890 | 2 | 4 | 1 | 5/2/2012 | 500 | TRUE |
| 5 | CompanyE | No.5 Street | 123890 | 2 | 4 | 1 | 5/2/2012 | 500 | FALSE |
| 1 | CompanyA | No.1 Street | 123456 | 3 | 1 | 5 | 9/10/2013 | 3069 | TRUE |
| 2 | CompanyB | No.2 Street | 23453 | 3 | 1 | 5 | 9/10/2013 | 3069 | FALSE |
| 3 | CompanyC | No.3 Street | 45321 | 3 | 1 | 5 | 9/10/2013 | 3069 | FALSE |
| 4 | CompanyD | No.4 Street | 4567890 | 3 | 1 | 5 | 9/10/2013 | 3069 | FALSE |
| 5 | CompanyE | No.5 Street | 123890 | 3 | 1 | 5 | 9/10/2013 | 3069 | FALSE |
由于用的是 INNER JOIN,所以不考虑 T1(T2)中存在而T2(T1)中不存在的情况，继续执行笛卡尔乘积和 ON 条件筛选，得到结果如下表：
| ID | Name | Adress | PhoneNumber | ID | Cus_ID | Product_ID | OrderDate | Qua | ID | Name | Price |
| 4 | CompanyD | No.4 Street | 4567890 | 2 | 4 | 1 | 5/2/2012 | 500 | 1 | ABC | 10 |
| 2 | CompanyB | No.2 Street | 23453 | 1 | 2 | 2 | 1/1/2012 | 1000 | 2 | DEF | 16 |
| 1 | CompanyA | No.1 Street | 123456 | 3 | 1 | 5 | 9/10/2013 | 3069 | 5 | MNO | 100 |
由于没有用到 WHERE 条件，没有 GROUP BY，没有 HAVING 之类的操作。直接执行 SELECT 中需要选中的列，返回数据查询结果。
| CustomerName | OrderDate | Qua | ProductName | Price |
| CompanyD | 5/2/2012 | 500 | ABC | 10 |
| CompanyB | 1/1/2012 | 1000 | DEF | 16 |
| CompanyA | 9/10/2013 | 3069 | MNO | 100 |
至此，SQL 的逻辑执行过程处理完毕。
SQL 查询基础(2)-实例,码迷,mamicode.com
标签：style blog http java color 数据
原文地址：http://www.cnblogs.com/Danielhu1983/p/3698690.html
