how to select or distignuish two rows in the same table with the same value on one field but different on another

Question

I know the question is too long but i cant describe it simple enough. So here is my case:

I want to select two or more rows in a same table in mysql with one of their field has the same value but will vary on the other field.

Example:

mainthread |  category

1234             1
1234             1
1234             3
1234             3
5643             4
4322             6
3123             9

now i want all the category of 1 will be updated to 3 when they have the same mainthread value. The category i want to change is always from 1 to 3. It's the mainthread that will vary cause there's like thousands of different mainthreads

Answer 1

Maybe I'm misunderstanding the question, but isn't this just:

Update mytable
Set category = 3
Where mainthread = 1234

Answer 2

If you want to update the table, to set the value of category to be the same on all the rows with the same value for mainthread , then

UPDATE mytable t
  JOIN ( SELECT mainthread
              , MAX(category) AS max_category
           FROM mytable
          GROUP BY mainthread
       ) s
    ON s.mainthread = t.mainthread
  SET t.category = s.max_category

If you just want to return rows that have the same mainthread value with two or more different values for category, then:

SELECT t.*
  FROM mainthread t
  JOIN ( SELECT mainthread
           FROM mytable
          GROUP BY mainthread
         HAVING COUNT(DISTINCT category) > 1
       ) s
    ON s.mainthread = t.mainthread
 ORDER
    BY t.mainthread
     , t.category

(It's not entirely clear from your question what you are trying to accomplish.)