MySQL -获取x条记录的每一个值y在某一列 - MySQL - Get x records for every value y in a certain column

MySQL - Get x records for every value y in a certain column

我有一个这样的表:

列:

id | parent

记录:

对于所有的家长，我想要得到前2条记录，所以我应该有:

如何在MySQL中实现这一点?我需要使用子查询?谢谢。

try this

  SELECT a.id,  a.parent FROM Table1 AS a WHERE 
 (SELECT COUNT(*) FROM Table1 AS b 
 WHERE b.parent = a.parent AND b.id <= a.id) <= 2 
 ORDER BY a.parent ASC, a.id asc

演示

 ID     PARENT
 1       1
 2       1
 4       2
 5       2
 7       3
 8       4
 9       5

这里有几个不同的解决方案可以尝试。

SELECT id, parent
FROM (
    SELECT id, (@rownum:=IF(parent=@parent, @rownum+1, 1)) AS rownum, 
      (@parent:=parent) AS parent
    FROM (SELECT @parent := null) AS _init
    STRAIGHT_JOIN MyTable
    ORDER BY parent) AS t
WHERE t.rownum <= 2;

替代:

SELECT t1.id, t1.parent
FROM MyTable AS t1
LEFT OUTER JOIN MyTable AS t2
  ON t1.parent = t2.parent AND t1.id >= t2.id
GROUP BY t1.id
HAVING COUNT(*) <= 2;