有条件的工会执行第二查询，如果第一次犯规返回行

Question

我有如下一个MySQL查询：

select m.field1, l.field2... l.fieldn 
from mytable m, logtable l 
where <some conditions to join m and l> 

union 

select m.field1, h.field2... h.fieldn 
from mytable m, loghistorytable h 
where <some conditions to join m and h> 

现在既然历史表是相当巨大的，命中的表现，我想执行从历史上第二只选择如果当前日志表没有返回数据。我可以修改查询吗？

Answer 1

尝试重复第一个查询在第二个查询的条件如下：

select m.field1, l.field2... l.fieldn 
from mytable m, logtable l 
where <some conditions to join m and l> 

union 

select m.field1, h.field2... h.fieldn 
from mytable m, loghistorytable h 
where <some conditions to join m and h> 
and not exists (select m.field1, l.field2... l.fieldn 
       from mytable m, logtable l 
       where <some conditions to join m and l>)