sql语句from (select distinct Subject from tb) as什么意思

Question

declare @sql varchar(8000)set @sql = 'select Name as ' + '姓名'select @sql = @sql + ' , max(case Subject when ''' + Subject + ''' then Result else 0 end) [' + Subject + ']'from (select distinct Subject from tb) as aset @sql = @sql + ' from tb group by name'exec(@sql)from (select distinct Subject from tb) as aas a必须要么。可不可以不要

Answer 1

把子查询：
select distinct Subject from tb
的结果作为一个表a来使用（理解成临时视图a也可以）

关于sql语句中 from 子表伪名的问题：

Oracle支持：
select * from (xxx)
select * from (xxx) a

SQL Server和db2支持：
select * from (xxx) as a
select * from (xxx) a

sybase 都不支持，另有写法

所以，最好写成：
select * from (xxx) a