SELECT sum(c) from (
select count(*) as c from table1
UNION ALL
select count(*) as c from table2
UNION ALL
select count(*) as c from table3
UNION ALL
select count(*) as c from table4
)
union(或称为联合)的作用是将多个结果合并在一起显示出来。 union和union all的区别是,union会(去重
)自动压缩多个结果集合中的重复结果,而union all则(不去重
)将所有的结果全部显示出来,不管是不是重复