java - 資料庫查詢多表

Question

前提 oralce，mybatis
現在有多張表
我現在需要統計每張表裡面的資訊的數量,也就是count(*)

我現在的方法是寫了多個方法
例如 mapper裡：
long selectCountA;
long selectCountB;
long selectCountC;

這樣的話，我要去資料庫裡查三次。分別獲得3個資料
我想能不能 寫一句sql語句 直接得到三個值

求解？

。能給我一個oracle語句的嗎， 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

採用多列的寫法

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

這樣?

Answer 3

Mysql

Oracle
在以上語句後面加 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也有類似的系統表