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_2

123       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_1

123

456

Answer 1

SELECT  (        SELECT COUNT(*)        FROM   tab1        ) AS count1,        (        SELECT COUNT(*)        FROM   tab2        ) AS count2FROM    dual

Answer 2

只是因为它略有不同：SELECT 'table_1' AS table_name, COUNT(*) FROM table_1UNIONSELECT 'table_2' AS table_name, COUNT(*) FROM table_2UNIONSELECT '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,       c2from   t1_count,       t2_count/select c1,       c2from   (select count(*) c1 from t1) t1_count,       (select count(*) c2 from t2) t2_count/