java - 数据库查询多表

Question

前提 oralce，mybatis
现在有多张表
我现在需要统计每张表里面的信息的数量,也就是count(*)

我现在的方法是写了多个方法
比如 mapper里：
long selectCountA;
long selectCountB;
long selectCountC;

这样的话，我要去数据库里查三次。分别获得3个数据
我想能不能 写一句sql语句 直接获得三个值

求解？

。能给我一个oracle语句的吗， 咋都是mysql。。

Answer 1

select "a" name, count(1)
from tableA
union
select "b" name, count(1)
from tableB
union
select "C" name, count(1)
from tableC

采用多列的写法

with 
temp_a as (select count(*) num from talbeA),
temp_b as (select count(*) num from tableB),
temp_c as (select count(*) num from tableC)
select temp_a.num, temp_b.num, temp_c.num from dual;

Answer 2

select A.countA,B.countB from (select count(*) as countA from t_countA) as A ,(select count(*) as countB from t_countB) as B

这样?

Answer 3

Mysql

Oracle
在以上语句后面加 from dual

Answer 4

Mysql的
select table_rows from information_schema.TABLES where table_schema in ('schema1','schema2','scheman') and table_name in ('tableName1','tableName2','tableNameN')
相信 oralce也有类似的系统表