“未知列”,因为子查询中的子查询

使用派生表/内联视图和表别名:

  SELECT product_id AS id,
         GROUP_CONCAT (y.value ORDER BY y.`order`) 
    FROM slud_products t1 
    JOIN (SELECT sd.product_id, 
                 sd.value,
                 sd.`order`
            FROM SLUD_DATA sd 
       LEFT JOIN slud_types ON slud_types.type_id = slud_data.type_id 
           WHERE value! = '' 
             AND display = 0) y ON y.product_id = t1.product_id 
                               AND y.order <= 3
   WHERE now() < DATE_ADD(date,INTERVAL +ttl DAY) 
     AND activated = 1
GROUP BY product_id
ORDER BY t1.date DESC

我发现在深度超过2个选择的查询中使用变量更为方便。

SELECT *,@product := product_id id,
    (SELECT GROUP_CONCAT (value ORDER By `order` ASC SEPARATOR ', ') 
    FROM (
        SELECT `order`,value 
        FROM slud_data 
        LEFT JOIN slud_types ON slud_types.type_id=slud_data.type_id 
        WHERE slud_data.product_id = @product
          AND value!='' AND display=0 
        LIMIT 3
    ) tmp) text
FROM slud_products t1 
WHERE 
    now() < DATE_ADD(date,INTERVAL +ttl DAY) AND activated=1
    ORDER BY t1.date DESC