SELECT
    id,
    tag,
    user_id,
    table_id,
    key_id,
    tag,
    `class`,
    COUNT(SELECT subTag.tag FROM `tags` AS subTag WHERE subTag.tag = curRow.tag)
FROM
    `tags` AS curRow
WHERE
    curRow.table_id = 'application'
AND
    ((
        curRow.user_id = 500
    OR
        curRow.class = 0
    ))
;