Inside Microsoft SQL Server 2005：T-SQL Querying （逻辑查询处理顺序|第一章）
如下所示SQL语句展示了逻辑查询的处理顺序
(8) SELECT (9) DISTINCT (11)
(1) FROM
(3) JOIN
(2) ON
(4) WHERE
(5) GROUP BY
(6) WITH {CUBE | ROLLUP}
(7) HAVING
(10) ORDER BY
步骤1 FROM：查询语句中from语句中的两个表进行交叉连接，生成虚拟表V1
步骤2 ON：筛选器ON应用到V1上，并把满足条件的行插入虚拟表V2
步骤3 OUTER（join）：如果SQL语句中选择了OUTER JOIN则将其余的行也插入V2，生成虚拟表V3,。如果join语句中存在多个table，则重复以上步骤。
步骤4 WHERE：根据条件筛选相应的行插入至虚拟表V4
步骤5 GROUP BY：分类V4并插入虚拟表V5
步骤6 CUBE | ROLLUP：增加超级组的行至V5，生成V6
步骤7 HAVING：根据条件筛选相应的行插入至虚拟表V7
步骤8 SELECT：生成SELECT内容，生成V8
步骤9 DISTINCT：去掉重复的行，生成V9
步骤10 ORDER BY：排序生成V10
步骤11 TOP：保留前几条记录并返回结果
我们可以通过一个具体实例说明问题
Table 1如下
| CustomerID | City |
| FISSA | Madrid |
| FRNDO | MAdrid |
| KRLOS | Madrid |
| MRPHS | Zion |
Table 2如下
| OrdID | CustomerID |
| 1 | FRNDO |
| 2 | FRNDO |
| 3 | KRLOS |
| 4 | KRLOS |
| 5 | KRLOS |
| 6 | MRPHS |
| 7 | NULL |
需要使用的query语句如下：
SELECT C.customerid, COUNT(O.orderid) AS numorders
FROM dbo.Customers AS C
LEFT OUTER JOIN dbo.Orders AS O
ON C.customerid = O.customerid
WHERE C.city = 'Madrid'
GROUP BY C.customerid
HAVING COUNT(O.orderid) < 3
ORDER BY numorders;
第一步，两个table连接并生产一个28行的虚拟表V1（4*7），如下
| C.customerid | C.city | O.orderid | O.customerid |
| FISSA | Madrid | 1 | FRNDO |
| FISSA | Madrid | 2 | FRNDO |
| FISSA | Madrid | 3 | KRLOS |
| FISSA | Madrid | 4 | KRLOS |
| FISSA | Madrid | 5 | KRLOS |
| FISSA | Madrid | 6 | MRPHS |
| FISSA | Madrid | 7 | NULL |
| FRNDO | Madrid | 1 | FRNDO |
| FRNDO | Madrid | 2 | FRNDO |
| FRNDO | Madrid | 3 | KRLOS |
| FRNDO | Madrid | 4 | KRLOS |
| FRNDO | Madrid | 5 | KRLOS |
| FRNDO | Madrid | 6 | MRPHS |
| FRNDO | Madrid | 7 | NULL |
| KRLOS | Madrid | 1 | FRNDO |
| KRLOS | Madrid | 2 | FRNDO |
| KRLOS | Madrid | 3 | KRLOS |
| KRLOS | Madrid | 4 | KRLOS |
| KRLOS | Madrid | 5 | KRLOS |
| KRLOS | Madrid | 6 | MRPHS |
| KRLOS | Madrid | 7 | NULL |
| MRPHS | Zion | 1 | FRNDO |
| MRPHS | Zion | 2 | FRNDO |
| MRPHS | Zion | 3 | KRLOS |
| MRPHS | Zion | 4 | KRLOS |
| MRPHS | Zion | 5 | KRLOS |
| MRPHS | Zion | 6 | MRPHS |
| MRPHS | Zion | 7 | NULL |
第二步，判断ON的条件生产虚拟表V2，如下
| C.customerid | C.city | O.orderid | O.customerid |
| FRNDO | Madrid | 1 | FRNDO |
| FRNDO | Madrid | 2 | FRNDO |
| KRLOS | Madrid | 3 | KRLOS |
| KRLOS | Madrid | 4 | KRLOS |
| KRLOS | Madrid | 5 | KRLOS |
| MRPHS | Zion | 6 | MRPHS |
第三步，添加OUTER JOIN的结果，如下
| C.customerid | C.city | O.orderid | O.customerid |
| FRNDO | Madrid | 1 | FRNDO |
| FRNDO | Madrid | 2 | FRNDO |
| KRLOS | Madrid | 3 | KRLOS |
| KRLOS | Madrid | 4 | KRLOS |
| KRLOS | Madrid | 5 | KRLOS |
| MRPHS | Zion | 6 | MRPHS |
| FISSA | Madrid | NULL | NULL |
第四步，执行WHERE条件判断，如下
| C.customerid | C.city | O.orderid | O.customerid |
| FRNDO | Madrid | 1 | FRNDO |
| FRNDO | Madrid | 2 | FRNDO |
| KRLOS | Madrid | 3 | KRLOS |
| KRLOS | Madrid | 4 | KRLOS |
| KRLOS | Madrid | 5 | KRLOS |
| FISSA | Madrid | NULL | NULL |
第五步，执行GROUP排序，如下
| C.customerid | C.customerid | C.city | O.orderid | O.customerid |
| FRNDO | FRNDO | Madrid | 1 | FRNDO |
| FRNDO | Madrid | 2 | FRNDO | |
| KRLOS | KRLOS | Madrid | 3 | KRLOS |
| KRLOS | Madrid | 4 | KRLOS | |
| KRLOS | Madrid | 5 | KRLOS | |
| FISSA | FISSA | Madrid | NULL | NULL |
第六步，本列中略过
第七步，执行HAVING条件，如下
| C.customerid | C.customerid | C.city | O.orderid | O.customerid |
| FRNDO | FRNDO | Madrid | 1 | FRNDO |
| FRNDO | Madrid | 2 | FRNDO | |
| FISSA | FISSA | Madrid | NULL | NULL |
第八步，执行SELECT，如下
| C.customerid | numorders |
| FRNDO | 2 |
| FISSA | 0 |
第九步，执行DISTINCT
第十步，执行ORDER BY，如下
| C.customerid | numorders |
| FISSA | 0 |
| FRNDO | 2 |
第十一步，执行TOP