Bagaimana saya bisa memilih bidang teks terpanjang saat menggunakan GROUP BY di mysql, a la MAX()?

Anda perlu menggunakan CHAR_LENGTH bukannya 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

Anda mungkin ingin melihat perbedaannya:CHAR_LENGTH() vs PANJANG ( )

Demo SQLFiddle