删除具有相同ID的不同行上的多个位置-MySQL

Question

I have the following table structure that define the relationship between posts and their categories.

Post_id | Category_id
---------------------
1       | A
1       | B
1       | C
2       | A
2       | B
2       | C
2       | D

So i want to search the post that have two given categories (For example A and B) and then delete the row A. So the result would be:

Post_id | Category_id
---------------------
1       | B
1       | C
2       | B
2       | C
2       | D

How can achieve this?

Answer 1

Try this:

delete t1 
from yourtable t1
join (
    -- This sub query will retrieve records that have both A and B for each `Post_id`
    select Post_id
    from yourtable
    where Category_id in ('A', 'B')
    group by Post_id
    having count(distinct Category_id) = 2
) t2
-- Then join this sub query on your table.
on t1.Post_id = t2.Post_id
where t1.Category_id = 'A'

Demo Here

Answer 2

You can find the posts by doing:

select post_id
from post_categories
where category_id in ('A', 'B')
group by post_id
having count(distinct category_id) = 2;

You can then delete them with a join :

delete pc
    from post_categories pc join
         (select post_id
          from post_categories
          where category_id in ('A', 'B')
          group by post_id
          having count(distinct category_id) = 2
         ) todelete
         on pc.post_id = todelete.post_id
    where pc.category_id = 'A';