e_i_pi

我相信你想要的是这样的:

SELECT *
FROM (
    SELECT
        reg.id,
        reg.collection_id,
        cr.number,
        COUNT(*) AS ct
    FROM tbl_registers AS reg
    INNER JOIN tbl_cross AS cr
        ON cr.register_id = reg.id
    GROUP BY
        reg.id,
        reg.collection_id,
        cr.number
) duplicates
WHERE duplicates.ct > 1
-- AND duplicates.collection_id = 56