从多个表中选择count（*）

Question

如何count(*)从具有结果的两个不同表（分别称为tab1和tab2）中进行选择：Count_1   Count_2123       456我已经试过了：select count(*) Count_1 from schema.tab1 union all select count(*) Count_2 from schema.tab2但是我所拥有的是：Count_1123456

Answer 1

SELECT  (

        SELECT COUNT(*)

        FROM   tab1

        ) AS count1,

        (

        SELECT COUNT(*)

        FROM   tab2

        ) AS count2

FROM    dual

Answer 2

只是因为它略有不同：

SELECT 'table_1' AS table_name, COUNT(*) FROM table_1

UNION

SELECT 'table_2' AS table_name, COUNT(*) FROM table_2

UNION

SELECT 'table_3' AS table_name, COUNT(*) FROM table_3

它给出了换位的答案（每张表一行而不是一列），否则我认为这没有太大不同。我认为在性能方面，它们应该是等效的。

Answer 3

其他略有不同的方法：

with t1_count as (select count(*) c1 from t1),

     t2_count as (select count(*) c2 from t2)

select c1,

       c2

from   t1_count,

       t2_count

/

select c1,

       c2

from   (select count(*) c1 from t1) t1_count,

       (select count(*) c2 from t2) t2_count

/