原来，union的优先级很高。比如
select stdname,[subject]='化学',化学 as score from #student2
union all
select stdname,[subject]='数学',数学 as score from #student2
union all
select stdname,[subject]='物理',物理 as score from #student2
union all
select stdname,[subject]='语文',语文 as score from #student2
order by stdname,[subject]
等同于
select * from(
select stdname,[subject]='化学',化学 as score from #student2
union all
select stdname,[subject]='数学',数学 as score from #student2
union all
select stdname,[subject]='物理',物理 as score from #student2
union all
select stdname,[subject]='语文',语文 as score from #student2
) as a
order by stdname,[subject]
\
