select (select count(*) from tab1) count_1, (select count(*) from tab2) count_2 from dual;

SELECT  (
        SELECT COUNT(*)
        FROM   tab1
        ) AS count1,
        (
        SELECT COUNT(*)
        FROM   tab2
        ) AS count2
FROM    dual

因为我找不到其他答案了。

如果你不喜欢子查询并且在每个表中都有主键，你可以这样做:

select count(distinct tab1.id) as count_t1,
       count(distinct tab2.id) as count_t2
    from tab1, tab2

但是就性能而言，我认为Quassnoi的解决方案更好，也是我会使用的解决方案。

作为附加信息，要在SQL Server中完成同样的事情，您只需要删除查询的“FROM dual”部分。

如果表(或者至少是一个键列)是相同类型的，那么就先做联合，然后计数。

select count(*) 
  from (select tab1key as key from schema.tab1 
        union all 
        select tab2key as key from schema.tab2
       )

或者把你的语句加上另一个和()。

select sum(amount) from
(
select count(*) amount from schema.tab1 union all select count(*) amount from schema.tab2
)

SELECT (SELECT COUNT(*) FROM table1) + (SELECT COUNT(*) FROM table2) FROM dual;