Mysql 如果第一个查询未返回行，则条件联合执行第二个查询

我有一个mysql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>

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>)

---

现在，由于历史记录表非常庞大，而且会影响性能，所以我只想在当前日志表不返回数据的情况下执行第二个select from history。我可以以某种方式修改查询吗？

尝试在第二个查询的条件下重复第一个查询，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>

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>)

---

今天的提示：切换到现代的显式连接语法！更容易编写而不出错，更容易读取和维护，并且在需要时更容易转换为外部联接。为什么不简单地执行查询1呢。如果没有结果，则执行查询2。