我假设您正在寻找最高的值，或者通过优先选择queue\u customer而不是queue\u number（使用coalesce…），或者通过想要两个值中的最高值（使用if（coalesce…）。

SELECT COALESCE( c.your_number, n.queue_current_number ) 
    /* or maybe you want IF( COALESCE( c.your_number, 0 ) > COALESCE( n.queue_current_number, 0 ), c.your_number, n.queue_current_number ) */
    FROM shop s
    LEFT JOIN queue_number n   ON s.id1 = n.id_shop 
                              AND NOT EXISTS( SELECT * FROM queue_number n2
                                              WHERE n.id_shop = n2.id_shop
                                                AND n.queue_current_number < n2.queue_current_number )
    LEFT JOIN queue_customer c ON n.id2 = c.id_queue
                              AND NOT EXISTS( SELECT * FROM current_number c2
                                              WHERE c.id_queue = c2.id_queue
                                                AND n.your_number < n2.your_number )
    WHERE s.id_shop = 1

使用子查询尝试以下操作：

SELECT shop.id1, shop.name, COALESCE(younumber,queue_current_number) as q_current_number
FROM shop
LEFT JOIN queue_number ON queue_number.id_shop = shop.id1
left join
(select id_queue,max(your_number) as younumber
from queue_customer group by id_queue)c on c.id_queue= shop.id1
WHERE id_shop = 1