T-SQL查询处理执行顺序（一）

[转自]http://www.cnblogs.com/gaiyang/archive/2011/04/13/2014355.html

对于T-SQL编程，用得最广泛的，莫过于查询（Querying）。要想写出高质量、高性能的查询语句，必须深入地了解逻辑查询处理。

一、逻辑查询处理的各个阶段

(5)SELECT DISTINCT TOP(<top_specification>) <select_list>

(1)FROM <left_table> <join_type> JOIN <right_table> ON <on_predicate>

(2)WHERE <where_predicate>

(3)GROUP BY <group_by_specification>

(4)H***ING <having_predicate>

(6)ORDER BY <order_by_list>

上边语句是一个普通格式的查询语句，基本包含了所有的查询条件和关键字。你可能会发现前边的序号并不是按顺序来的，被你说对了，这是SQL与其他编程语言不同的最明显特征，就是它的执行顺序并不是按照编写顺序来的。上边的序号，就是查询语句在执行过程中的逻辑处理顺序。下面简单介绍一下各个阶段都干了啥事。

（1）FROM 阶段

FROM阶段标识出查询的来源表，并处理表运算符。在涉及到联接运算的查询中（各种join），主要有以下几个步骤：

　　a.求笛卡尔积。不论是什么类型的联接运算，首先都是执行交叉连接（cross join），求笛卡儿积，生成虚拟表VT1-J1。

b.ON筛选器。这个阶段对上个步骤生成的VT1-J1进行筛选，根据ON子句中出现的谓词进行筛选，让谓词取值为true的行通过了考验，插入到VT1-J2。

c.添加外部行。如果指定了outer join，还需要将VT1-J2中没有找到匹配的行，作为外部行添加到VT1-J2中，生成VT1-J3。

经过以上步骤，FROM阶段就完成了。概括地讲，FROM阶段就是进行预处理的，根据提供的运算符对语句中提到的各个表进行处理（除了join，还有apply，pivot，unpivot）

（2）WHERE阶段

WHERE阶段是根据<where_predicate>中条件对VT1中的行进行筛选，让条件成立的行才会插入到VT2中。

（3）GROUP BY阶段

GROUP阶段按照指定的列名列表，将VT2中的行进行分组，生成VT3。最后每个分组只有一行。

（4）H***ING阶段

该阶段根据H***ING子句中出现的谓词对VT3的分组进行筛选，并将符合条件的组插入到VT4中。

（5）SELECT阶段

　　这个阶段是投影的过程，处理SELECT子句提到的元素，产生VT5。这个步骤一般按下列顺序进行

a.计算SELECT列表中的表达式，生成VT5-1。

b.若有DISTINCT，则删除VT5-1中的重复行，生成VT5-2

c.若有TOP，则根据ORDER BY子句定义的逻辑顺序，从VT5-2中选择签名指定数量或者百分比的行，生成VT5-3

（6）ORDER BY阶段

根据ORDER BY子句中指定的列明列表，对VT5-3中的行，进行排序，生成游标VC6.

当然SQL SERVER在实际的查询过程中，有查询优化器来生成实际的工作计划。以何种顺序来访问表，使用什么方法和索引，应用哪种联接方法，都是由查询优化器来决定的。优化器一般会生成多个工作计划，从中选择开销最小的那个去执行。逻辑查询处理都有非常特定的顺序，但是优化器常常会走捷径。
下一篇文章:Tsql查询执行顺序（二）[揭露join中on和where的区别]

--------------------------------------------------另个详细的原文--------------------------------------------------------

以下来自《Microsoft.Press.Inside.Microsoft.SQL.Server.2005.T-SQL.Querying.Apr.2006.chm》：

(8)  SELECT (9) DISTINCT (11) <TOP_specification> <select_list>

(1)  FROM <left_table>

(3)    <join_type> JOIN <right_table>

(2)      ON <join_condition>

(4)  WHERE <where_condition>

(5)  GROUP BY <group_by_list>

(6)  WITH {CUBE | ROLLUP}

(7)  H***ING <having_condition>

(10) ORDER BY <order_by_list>

Brief Description of Logical Query Processing Phases

Don't worry too much if the description of the steps doesn't seem to make much sense for now. These are provided as a reference. Sections that come after the scenario example will cover the steps in much more detail.

1.	FROM: A Cartesian product (cross join) is performed between the first two tables in the FROM clause, and as a result, virtual table VT1 is generated.
2.	ON: The ON filter is applied to VT1. Only rows for which the <join_condition> is TRUE are inserted to VT2.
3.	OUTER (join): If an OUTER JOIN is specified (as opposed to a CROSS JOIN or an INNER JOIN), rows from the preserved table or tables for which a match was not found are added to the rows from VT2 as outer rows, generating VT3. If more than two tables appear in the FROM clause, steps 1 through 3 are applied repeatedly between the result of the last join and the next table in the FROM clause until all tables are processed.
4.	WHERE: The WHERE filter is applied to VT3. Only rows for which the <where_condition> is TRUE are inserted to VT4.
5.	GROUP BY: The rows from VT4 are arranged in groups based on the column list specified in the GROUP BY clause. VT5 is generated.
6.	CUBE | ROLLUP: Supergroups (groups of groups) are added to the rows from VT5, generating VT6.
7.	H***ING: The H***ING filter is applied to VT6. Only groups for which the <having_condition> is TRUE are inserted to VT7.
8.	SELECT: The SELECT list is processed, generating VT8.
9.	DISTINCT: Duplicate rows are removed from VT8. VT9 is generated.
10.	ORDER BY: The rows from VT9 are sorted according to the column list specified in the ORDER BY clause. A cursor is generated (VC10).
11.	TOP: The specified number or percentage of rows is selected from the beginning of VC10. Table VT11 is generated and returned to the caller.