与不同的表进行JOIN

SELECT COUNT(*) FROM (  
SELECT DISTINCT table_a.ID  FROM table_a JOIN table_c ON table_a.ID  = table_c.ID   );

select (select count(*) from tab1) count_1, (select count(*) from tab2) count_2 from dual;

作为附加信息，要在SQL Server中完成同样的事情，您只需要删除查询的“FROM dual”部分。

选择 (select count() from tab1 where field like 'value') + (select count() from tab2 where field like 'value') 数

    select 
    t1.Count_1,t2.Count_2
    from 
(SELECT count(1) as Count_1 FROM tab1) as t1, 
(SELECT count(1) as Count_2 FROM tab2) as t2

SELECT  (
        SELECT COUNT(*)
        FROM   tab1
        ) AS count1,
        (
        SELECT COUNT(*)
        FROM   tab2
        ) AS count2
FROM    dual