MYSQL UPDATE with IN and Subquery

Question

Hi i have tables like this :

table entry :

id | total_comments
___________
1 | 0
2 | 0
3 | 0
4 | 0

table comments :

id | eid | comment
___________
1 | 1 | comment sdfd
2 | 1 | testing testing
3 | 1 | comment text
4 | 2 | dummy comment
5 | 2 | sample comment
6 | 1 | fg fgh dfh

Query i write :

UPDATE entry 
   SET total_comments = total_comments + 1 
 WHERE id IN ( SELECT eid 
                 FROM comments 
                WHERE id IN (1,2,3,4,5,6))

Results i get is :

table entry :

id | total_comments
___________
1 | 1
2 | 1
3 | 0
4 | 0

Expected results :

table entry :

id | total_comments
___________
1 | 4
2 | 2
3 | 0
4 | 0

Any help will be appreciated.

Answer 1

That's exactly what I'd expect. The id is IN the set you give it, so total_comments = total_comments + 1.

It's not going to add one for each instance of the same value: that's not how IN works. IN will return a simple boolean yes/no.

Answer 2

Try:

UPDATE entry
  SET total_comments = (SELECT COUNT(*) 
                        FROM comments
                        WHERE entry.id = comments.eid
                        GROUP BY id)

Answer 3

If you really need total_comments in a separate table, I would make that a VIEW.

CREATE VIEW entry AS SELECT id, count(comments) as total_comment FROM comments GROUP by id

This way you avoid the maintenance task of updating the total_comments table altogether.

Answer 4

Use:

UPDATE entry 
   SET total_comments = (SELECT COUNT(*)
                           FROM COMMENTS c
                          WHERE c.eid = id
                       GROUP BY c.eid)
 WHERE id IN ( SELECT eid 
                 FROM comments 
                WHERE id IN (1,2,3,4,5,6))

Answer 5

UPDATE entry e 
    SET total_comments = ( SELECT COUNT(*) FROM comments WHERE eid = e.id)  
    WHERE 
    e.id in (SELECT eid FROM comments WHERE id IN (1,2,3,4,5,6))