mysql统计多张表中的总数

需求：mysql统计多张表中的总数

因为需求不明确，答案有三种：

答案1：

select sum(a) from (select count(*) a from tb1 union select count(*) a from tb2 union select count(*) a from tb3) as tb;

答案2：

select count(*) a from tb1 union select count(*) b from tb2 union select count(*) c from tb3;

答案3：

select a.countNum,b.countNum,c.countNum

from(

    (select COUNT(*) as countNum from tb1) a,

    (select COUNT(*) as countNum from tb2) b,

    (select COUNT(*) as countNum from tb3) c

)