先对结果集UNION再做LEFT JOIN,与先做LEFT JOIN再做UNION,检索出的结果集会不同么,如果相同哪个性能好呢!!急急急select
a.1 as aa,
a.2 as bb,
b.3 as cc
from
a left join b on (a.4=b.4)
union
select
c.1 as aa,
c.2 as bb,
b.3 as cc
from
c left join b on (c.4=b.4)与
select
d.aa,
d.bb,
b.3 as cc
from
(select
a.1 as aa,
a.2 as bb
from
a
union
select
c.1 as aa,
c.2 as bb
from
c
)d left join b on (d.4=b.4)
a.1 as aa,
a.2 as bb,
b.3 as cc
from
a left join b on (a.4=b.4)
union
select
c.1 as aa,
c.2 as bb,
b.3 as cc
from
c left join b on (c.4=b.4)与
select
d.aa,
d.bb,
b.3 as cc
from
(select
a.1 as aa,
a.2 as bb
from
a
union
select
c.1 as aa,
c.2 as bb
from
c
)d left join b on (d.4=b.4)
解决方案 »
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