方法一：
select t1.num1,t2.num2,t3.num3 from
(select count(*) num1 from table1) t1,
(select count(*) num2 from table2) t2,
(select count(*) num3 from table3) t3
方法二：
select sum(t.num1),sum(t.num2),sum(t.num3) from (
select count(*) num1,0 as num2,0 as num3 from table1
union
select 0 as num1,count(*) num2,0 as num3 from table2
union
select 0 as num1,0 as num2,count(*) num3 from table3
) t