右側加入子查詢內部Mysql訪問主表

Question

SELECT id,region FROM ads  
RIGHT JOIN (SELECT * FROM ads a2 WHERE a2.region=ads.region LIMIT 4) AS regions  
ON regions.id=ads.id 

說「未知列ads.region」。如何訪問主表列區域？

我需要爲每個區域

Answer 1

使用取4行：

SELECT a.id, 
      a.region 
     FROM ads a  
RIGHT JOIN (SELECT t.*, 
        CASE 
         WHEN @region = t.region THEN @rownum := @rownum + 1 
         ELSE @rownum := 1 
        END as rank, 
        @region := t.region 
       FROM ADS t 
       JOIN (SELECT @rownum := 0, @region := 1) r 
      ORDER BY t.region) b ON b.id = a.id 
           AND b.region = a.region 
           AND b.rank <= 4