mysql - MySQL GROUP BY 和 HAVING

Question

我正在根据 X 列对结果进行分组，并且我想返回组中 Y 列值最高的行。

SELECT * 
FROM   mytable 
GROUP  BY col1 
HAVING col2 >= (SELECT MAX(col2) 
                FROM   mytable AS mytable2 
                WHERE  mytable2.col1 = mytable.col1 GROUP BY mytable2.col1) 

我想优化上面的查询。没有子查询是否可行？

我找到了解决方案，它比您想象的要简单：

SELECT * FROM (SELECT * FROM mytable ORDER BY col2 DESC) temp GROUP BY col1

在 20,000 行上运行 5 毫秒。

Answer 1

为 JOIN 使用派生表/内联视图：

SELECT x.* 
  FROM mytable x
  JOIN (SELECT t.col1,
               MAX(t.col2) AS max_col2
          FROM MYTABLE t
      GROUP BY t.col1) y ON y.col1 = x.col1
                        AND y.max_col2 >= x.col2

请注意，x如果有多个相关记录，这将重复记录y。要删除重复项，请使用DISTINCT：

SELECT DISTINCT x.* 
  FROM mytable x
  JOIN (SELECT t.col1,
               MAX(t.col2) AS max_col2
          FROM MYTABLE t
      GROUP BY t.col1) y ON y.col1 = x.col1
                        AND y.max_col2 >= x.col2

以下内容未经测试，但不会返回重复项（假设有效）：

SELECT x.* 
  FROM mytable x
 WHERE EXISTS (SELECT NULL
                 FROM MYTABLE y
                WHERE y.col1 = x.col1
             GROUP BY y.col1
               HAVING MAX(y.col2) >= x.col2)

Answer 2

Your Col2 never be > then MAX(col2) so i suggest to use col2 = MAX(col2)

so HERE is the QUERY

SELECT * FROM  mytable GROUP BY col1 HAVING  col2 = MAX(  col2 )