在多表联接中使用Group by执行Limit by和Max的Order by和Limit

• Group_Concat() 函数有一个可选的 Order By 子句,使用它,我们可以根据另一列/同一列对列值进行排序以进行concat。
• Substring_Index() 函数将连接的字符串限制为每组5个。

尝试:

SELECT Substring_Index(Group_concat(ID 
                                    ORDER BY LJ.Population ASC 
                                    SEPARATOR '|'), 
                       '|', 
                       5) 
FROM   TableJ AS J
       LEFT JOIN TableL AS L
              ON L.J_ID = J.ID
       LEFT JOIN TableB AS B
              ON B.LJ_ID = L.F_ID
       LEFT JOIN TableLJ AS LJ
              ON LJ.ID = L.F_ID
GROUP  BY J.ID 

在 MySQL 8.0.2 and onwards Window Functions ,以确定 Row_Number() J.ID . 我们可以分类 LJ.Population 以获得相应的行号。

现在,将此结果集用作 Derived Table Group By 和 Concat()组 因此。

SELECT   Group_Concat(dt.ID ORDER BY dt.rownumber ASC SEPARATOR '|')
FROM     (
          SELECT    J.ID,
                    ROW_NUMBER() OVER (PARTITION BY J.ID
                                       ORDER BY LJ.Population ASC) AS rownumber
          FROM      TableJ   AS J
          LEFT JOIN TableL   AS L
            ON  L.J_ID = J.ID
          LEFT JOIN TableB   AS B
            ON  B.LJ_ID = L.F_ID
          LEFT JOIN TableLJ  AS LJ
            ON  LJ.ID = L.F_ID ) AS dt
WHERE    dt.rownumber <= 5
GROUP BY dt.ID