SQL语句的解析过程

雨点打透心脏的1/2处 2022-09-26 03:48 91阅读 0赞

由于最近需要做一些sql query性能提升的研究，因此研究了一下sql语句的解决过程。本文是我在看了各种资料后收集总结的，会详细的，一步一步的讲述一个sql语句的各个关键字的解析过程，欢迎大家互相学习。

#### SQL语句的解析顺序 ####

简单的说一个sql语句是按照如下的顺序解析的：

*  1、FROM FROM后面的表标识了这条语句要查询的数据源。和一些子句如，（1-J1）笛卡尔积，（1-J2）ON过滤，（1-J3）添加外部列，所要应用的对象。FROM过程之后会生成一个虚拟表VT1。

*  (1-J1)笛卡尔积 这个步骤会计算两个相关联表的笛卡尔积(CROSS JOIN) ，生成虚拟表VT1-J1。  
    
 *  (1-J2)ON过滤 这个步骤基于虚拟表VT1-J1这一个虚拟表进行过滤，过滤出所有满足ON 谓词条件的列，生成虚拟表VT1-J2。  
    
 *  (1-J3)添加外部行  如果使用了外连接，保留表中的不符合ON条件的列也会被加入到VT1-J2中，作为外部行，生成虚拟表VT1-J3。

*  2、WHERE 对VT1过程中生成的临时表进行过滤，满足where子句的列被插入到VT2表中。  
    
 *  3、GROUP BY 这个子句会把VT2中生成的表按照GROUP BY中的列进行分组。生成VT3表。  
    
 *  4、HAVING 这个子句对VT3表中的不同的组进行过滤，满足HAVING条件的子句被加入到VT4表中。  
    
 *  5、SELECT 这个子句对SELECT子句中的元素进行处理，生成VT5表。

*  (5-1)计算表达式 计算SELECT 子句中的表达式，生成VT5-1  
    
 *  (5-2)DISTINCT 寻找VT5-1中的重复列，并删掉，生成VT5-2  
    
 *  (5-3)TOP 从ORDER BY子句定义的结果中，筛选出符合条件的列。生成VT5-3表  
    
 *  ORDER BY 从VT5-3中的表中，根据ORDER BY 子句的条件对结果进行排序，生成VC6表。

####     ####

#### 客户，订单的查询例子 ####

首先创建一个Customers表，插入如下数据：

创建一个Orders表，插入如下数据：

假如我们想要查询来自Madrid的，订单数小于3的客户，并把他们的订单数显示出来，结果按照订单数从小到大进行排序。

SELECT C.customerid, COUNT(O.orderid) AS numordersFROM dbo.Customers AS C  LEFT OUTER JOIN dbo.Orders AS O    ON C.customerid = O.customeridWHERE C.city = 'Madrid'GROUP BY C.customeridHAVING COUNT(O.orderid) < 3 ORDER BY numorders

查询结果为：

下面我们会详细的讲述sql是怎样计算出这个结果的：

#### FROM子句 ####

FROM子句标识了需要查询的表，如果指定了表操作，会从左到右的处理，每一个基于一个或者两个表的表操作都会返回一个输出表。左边表的输出结果会作为下一个表操作的输入结果。例如，交表相关的操作有 (1-J1)笛卡尔积，(1-J2)ON过滤器，(1-J3)添加外部列。FROM句子生成虚拟表VT1。

##### Step 1-J1：执行笛卡尔积(CROSS JOIN) #####

笛卡尔积会把左右两个表每一行所有可能的组合都列出来生成表VT1-J1，如果左表有m列，右表有n列，那么笛卡尔积之后生成的VT1-J1表将会有m×n列。

Step 1-J1这个步骤等价于执行：

SELECT * from Customers C  CROSS JOIN Orders O

执行结果为：（共有4×7列）

<table style="margin:0px 0px 10px; padding:0px; border-collapse:collapse; width:670px; max-width:100%; color:rgb(62,62,62); font-family:&quot;Helvetica Neue&quot;,Helvetica,&quot;Hiragino Sans GB&quot;,&quot;Microsoft YaHei&quot;,Arial,sans-serif; font-size:16px; line-height:25.6px; widows:1; word-wrap:break-word!important">    
 <tbody style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important"> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> C.customerid</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> C.city</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> O.orderid</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> O.customerid</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 1</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 2</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 3</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 4</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 5</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 6</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FISSA</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 7</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> NULL</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 1</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 2</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 3</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 4</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 5</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 6</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 7</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> NULL</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 1</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 2</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 3</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 4</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 5</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 6</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Madrid</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 7</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> NULL</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 1</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 2</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> FRNDO</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 3</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 4</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 5</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> KRLOS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 6</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; max-width:100%; word-wrap:break-word!important; height:15pt"> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> MRPHS</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> Zion</td> 
   <td align="right" style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> 7</td> 
   <td style="margin:0px; padding:1px 1px 3px; word-wrap:break-word!important; word-break:break-all; border:1px solid silver; max-width:100%; border-collapse:collapse; vertical-align:bottom"> NULL</td> 
  </tr> 
 </tbody> 
</table>

##### Step 1-J2：应用ON过滤，(JOIN 条件) #####

ON过滤条件是sql的三个过滤条件(ON,WHERE,HAVING)中最先执行的，ON过滤条件应用于前一步生成的虚拟表(VT1-J1)，满足ON过滤条件的行会被加入到虚拟表VT1-J2中。在应用了ON 过滤之后，生成的VT1-J2表如下所示：

##### Step 1-J3：添加外部列 #####

这个步骤只会出现在使用了外连接的情况。对于外连接(LEFT,RIGHT, or FULL)，你可以标记一个或者两个表作为保留表。作为保留表意味着你希望这个表里面的所有列都被返回，即使它里面的数据不满足ON子句的过滤条件。LEFT OUTER JOIN 把左边的表标记为保留表，RIGHTOUTER JOIN把右边的表作为保留表，FULL OUTER JOIN把两个表都标记为保留表.Step 1-J3为根据VT1-J2中的虚拟表，添加了保留表中不满足ON 条件的列，在未保留表中没有对应的列，因此标记为NULL。这个过程生成了虚拟表VT1-J3。

如果FROM子句中有多个表操作运算，sql会按照从左到右的顺序处理，左边生成的临时表结果作为右边表的输入表。

#### Step 2 WHERE 子句 ####

WHERE过滤被应用到前一步生成的临时表中，根据WHERE过滤条件生成临时表VT2。

注意：由于数据现在还没有被分组，因此现在你不能使用聚合运算-例如：你不能使用这样的句子 WHERE orderdate = MAX(orderdate)。另外你也不能使用SELECT子句中创建的变量别名，因为现在还没有处理SELECT子句-例如你不能写这样的句子：SELECT YEAR(orderdate) AS orderyear . . . WHERE orderyear > 2008.

应用这个过滤

>     WHERE C.city = 'Madrid'

这时生成的临时表VT2的内容如下：

在这个例子中，你需要在ON子句中使用ON C.customerid = O.customerid过滤，没有订单的客户在1-J2这一步中被过滤掉，但是在1-J3这一步中作为外部列又被加回来。但是，由于你只想返回来自Madrid的客户，因此你需要在WHERE子句中过滤城市（WHERE C.city = ‘Madrid’)，如果你放在ON过滤中，不属于Madrid的客户在添加外部列中会被添加回来。

关于ON 和 WHERE 的区别需要在这里说明一下，ON 和WHERE 的主要区别在于 ON 实在添加外部列之前进行过滤，WHERE 是在之后。ON过滤掉的列会在1-J3中添加回来。如果你不需要添加外部列，那么这两个过滤是相同的。

#### Step 3 GROUP BY子句 ####

这个子句会把前一步中生成的临时表中的数据进行分组，每一行都会分到并且只分到一个组里，生成虚拟表VT3。VT3表中包含了VT2表中所有的数据，和分组标识符。

这是生成的临时表VT3的内容如下：

sql最终返回的结果中，每一个分组必须只能返回一行(除非被过滤掉)，因此当一个sql语句中使用了GROUP BY时，在GROUP BY后面处理的子句，如SELECT，HAVING子句等，只能使用出现在GROUP BY后面的列，对于没有出现GROUP BY后面的列必须使用聚合函数(如 MAX ,MIN,COUNT,AVG等)，保证每一个GROUP只返回一行。

#### Step 4 HAVING子句 ####

HAVING子句用来过滤前一步生成的临时表，并且只作用于分组后的数据，满足HAVING条件的GROUP被添加到虚拟表VT4中。

当应用了这个过滤：

HAVING COUNT(O.orderid) < 3

之后，生成的VT4表内容如下：

需要注意的一点是，这里面使用的是COUNT(O.orderid)，而不是COUNT(\*)，由于这个查询中添加了外部列，COUNT方法会忽略NULL的列，导致出现了你不想要的结果。

#### Step 5 SELECT 子句 ####

尽管出现在sql语句的最前面，SELECT在第五步的时候才被处理，SELECT子句返回的表会最终返回给调用者。这个子句包含三个子阶段：(5-1)计算表达式，(5-2) 处理DISTINCT，(5-3)应用TOP过滤。

Step 5-1 计算表达式

SELECT子句中的表达式可以返回或者操作前一步表中返回的基本列。如果这个sql语句是一个聚合查询，在Step 3之后，你只能使用GROUP BY中的列，对不属于GROUP集合中的列必须使用聚合运算。不属于FROM表中基本列的必须为其起一个别名，如YEAR(orderdate) AS orderyear。

注意：在SELECT子句中创建的别名，不能在之前的Step中使用，即使在SELECT子句中也不能。原因是sql的很多操作是同时操作(all at once operation)，至于什么是all-at-once operation这里就不再介绍了。因此，SELECT子句中创建的别名只能在后面的子句中使用，如ORDER BY。例如：SELECT YEAR(orderdate) AS orderyear . . . ORDER BY orderyear。

在这个例子中：

SELECT C.customerid, COUNT(O.orderid) AS numorders

结果会得到一个虚拟表VT5-1：

Step 5-2:应用DISTINCT子句

如果sql语句中使用了DISTINCT，sql会把重复列去掉，生成虚拟表VT5-2。

Step 5-3:应用TOP选项

TOP选项是T-SQL提供的一个功能，用来表示显示多少行。基于ORDER BY子句定义的顺序，指定个数的列会被查询出来。这个过程生成虚拟表VT5-3。

正如上文提到的，这一步依赖于ORDER BY定义的顺序来决定哪些列应该显示在前面。如果你没有指定结果的ORDER BY顺序，也没有使用WITH TIES子句，每一次的返回结果可能会不一致。

在我们的例子中，Step 5-3被省略了，因为我们没有使用TOP关键字。

#### Step 6：ORDER BY子句 ####

前一步返回的虚拟表在这一步被排序，根据ORDER BY子句指定的顺序，返回游标VC6。ORDER BY子句也是唯一一个可以使用SELECT子句创建的别名的地方。

注意:这一步和之前不同的地方在于，这一步返回的结果是一个游标，而不是表。sql是基于集合理论的，一个集合没有对他的行定义顺序，它只是一个成员的逻辑集合，因此成员的顺序并不重要。带有ORDER BY子句的sql返回一个按照特定序列组织每一行的对象。ANSI 把这样的一个对象叫游标。理解这一点对你了解sql很重要。

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#### 上面的步骤如图所示: ####

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本书中主要内容是参照 Inside Microsoft SQL Server 2008:T-SQL Query，中的内容，大家如果想深入了解sql查询相关的知识，可以找这本书看看。

[Center]: /images/20220717/5a1af333f76e49c982bf70a70c2b1421.png