[英]MySQL - Get x records for every value y in a certain column
我有一張這樣的桌子:
列:
id | parent
記錄:
1 | 1
2 | 1
3 | 1
4 | 2
5 | 2
6 | 2
7 | 3
8 | 4
9 | 5
對於所有父母,我想獲得前2條記錄,所以我應該有:
1 | 1
2 | 1
4 | 2
5 | 2
7 | 3
8 | 4
9 | 5
如何在MySQL中實現? 我需要使用子查詢嗎? 謝謝。
嘗試這個
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
ID PARENT
1 1
2 1
4 2
5 2
7 3
8 4
9 5
您可以嘗試以下兩種不同的解決方案。
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;
替代方案:
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;
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