How can I select the longest text field when using GROUP BY in mysql, a la MAX()?

Question

In MySql you can use the MAX() function to get the highest value when using GROUP BY, how can I do the same thing to get the longest string of text?

Sample table:

id_|_post_id|_title__________|_body_____________________________________________
1  | ZXBF1J | Favorite Color | My favorite color is blue.
2  | ZXBF1J | Favorite Color | My favorite color is blue, no wait...
3  | ZXBF1J | Favorite Color | My favorite color is blue, no wait, yelloooow!
4  | AR3D47 | Quest          | To seek..
5  | AR3D47 | Quest          | To seek the Holy
6  | AR3D47 | Quest          | To seek the Holy Grail.

The tricky part is that I want to ORDER BY id ASC to see the oldest entries on the top, and I want to group by the post_id which is not something that I can use to ORDER, and get the longest body.

Sample query:

SELECT post_id, title, MAX(body) // obviously MAX() doesn't work here
FROM posts
GROUP BY post_id
ORDER BY id ASC

Desired output:

post_id|_title__________|_body_____________________________________________
ZXBF1J | Favorite Color | My favorite color is blue, no wait, yelloooow!
AR3D47 | Quest          | To seek the Holy Grail.

Again the key is to select the longest body while maintaining the order based on the id.

Answer 1

You need use CHAR_LENGTH instead of LENGTH

SELECT a.id, a.post_id, a.body
FROM posts a INNER JOIN
(
    SELECT post_ID, title, MAX(CHAR_LENGTH(body)) totalLength
    FROM posts
    GROUP BY post_ID, title
) b ON a.post_id = b.post_ID AND
        a.title = b.title AND
        CHAR_LENGTH(a.body) = b.totalLength

You might want to see the difference: CHAR_LENGTH( ) vs LENGTH( )

SQLFiddle Demo