Sql Server 学习之SELECT语句总结（一）
Sql Server 学习之SELECT语句总结（一） 查询处理的逻辑顺序 (返回结果列表( 01 ) FROM 表 表 (((((] SELECT语句的基本组件 Setp1：[FROM] from子句内可指定表的名称。在此步骤中，SQL Server会列出select语句使用的表，若指定join表则会一并列出，通 过交错
Sql Server 学习之SELECT语句总结（一）
查询处理的逻辑顺序
(返回结果列表(01) FROM 表 表 (((((]
SELECT语句的基本组件Setp1：[FROM]
from子句内可指定表的名称。在此步骤中，SQL Server会列出select语句使用的表，若指定join表则会一并列出，通 过交错排列的方式，将可能的组合放入VT1（第一个虚拟表内）。
Setp2：[ON]
比较VT1内的数据，当条件成立时将正确的的数据插入VT2。
Setp3：[Outer(join)]
如果定义了outer join 而非 cross join 或 inner join，即保存的表VT2内没有对应的记录依然会被当做外部记录
而形成VT3。如果from子句中表的个数超过两个，则setp1~3会在join的执行结果与from子句所标记的下一个表
间重复执行，直到所有的表处理完毕。
Setp4：[WHERE 过滤条件]
过滤VT3中的数据，将符合条件的列形成VT4。
Setp5: [Group by]
group by 子句可用于指定VT4的分组方式，执行的结果形成VT5。
Setp6：[CUBE|ROLLUP]
cube|rollup子句会对setp5中已经group by的数据在做进一步的group by 操作。如果将setp5比作数据的小计
值，服务器空间，那么setp6就是计算数据的总计值，而产生的数据会被插入到VT6。
Setp7: [HAVING]
判断VT6所完成的分组表中的字段是否符合having设置的条件。当符合该条件时，会将其插入到VT7。
Setp8: [SELECT]
通过select生成返回结果，仅选符合条件要求的字段列表生成VT8。
Setp9：[DISTINCT]
移除VT8的重复数据记录，香港服务器，将结果插入VT9。
Setp10：[Order by order_list [ASC|DESC] ]
指定VT9的排序方式（递增或递减），并将排序后的结果形成VT10。
Setp11：[TOP]
使用此关键字时，香港服务器，SQL SERVER会将VT10中符合条件的前n条数据插入到VT11，并返回给用户。
Setp12: [返回结果列表]
指定返回项目（如描述结果集的列，表达式，变量或常数），并以逗号分隔列表。
Setp13：[INTO新表名称]
可省略的自变量，方便用户以"返回结果列表"里的项目与数据类型结构创建新的表，并可使用"新表名称"所指定的
表名称。
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