java - 数据库查询多表

问题描述

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。。

问题解答

select 'a' name, count(1)from tableAunionselect 'b' name, count(1)from tableBunionselect '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;回答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?

Mysql

Oracle在以上语句后面加 from dual

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