mysql - 条件联合 - 如果第一个查询不返回行，则执行第二个查询

Question

我有一个 mysql 查询如下:

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>

现在，由于历史表非常庞大并且影响性能，因此我想仅在当前日志表不返回数据时才执行历史记录中的第二个选择。我可以以某种方式修改查询吗？

Answer 1

尝试在第二个查询的条件中重复第一个查询，如下所示:

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>)