SELECT
    u.name AS source,
    u2.name AS target
FROM
    board_posts AS p

/* Get user data for the post */
LEFT JOIN board_users AS u
ON u.id = p.author_id

/* Try to get user data for the user inside quotation marks */
LEFT JOIN board_users AS u2
ON u2.name = substring_index(
    substring_index(p.message, '[quote="', - 1),
    '"]',
    1
)
/* CHAR(7) is the unicode I used to separate different previous names */
OR LEFT(u2.other_names, LOCATE(CONCAT("", CHAR(7), ""), u2.other_names) - 1) = substring_index(substring_index(p.message, '[quote="', - 1), '"]', 1)

WHERE
    LOWER(p.message) LIKE '[quote="%'
ORDER BY
    p.id DESC
LIMIT 50