只說(shuō)明概念
select from a left join b on where b_id is null

或者
select from a where a_id not in (select a_id from b)

通過(guò)子查詢，試試這個(gè)

# table_a為A表名，table_b為B表名
SELECT a_id '活動(dòng)ID', title '活動(dòng)名稱' FROM table_a WHERE  a_id NOT IN (SELECT a_id FROM table_b WHERE user_id = 7);

SELECT a.* FROM a 
WHERE NOT EXISTS(SELECT b.ID from b where a. a_id = b.a_id AND b. user_id = 7)