Question

table :First
..............................................................................
id | ros | table | column | stool | date | places|
..............................................................................
1   12  5   6   a   2017-07-17  goa  |
2   12  5   6   b   2017-07-17  delhi|
..............................................................................


table :second

..........................................................................
id  | ros | name | email | phone | stos|address |date |
....................................................................................................
5      12    and   and@her.com  394924673464    6  fddsfds  2017-07-03 |
6      12    her     her@and.com    84838593894     6  fdafdfd  2017-07-04  |
....................................................................................................


Query like I am Using:
SELECT `p`.* FROM `first` `p` LEFT JOIN `second` as `st` ON `st`.`ros`=`p`.`ros` ORDER BY `id` DESC LIMIT 10;

我的问题是为什么它返回4行而不是2行？它没有在表格中恢复4值我是否需要使用distinct或group by？

Answer 1

尝试此查询：

SELECT p.* FROM first as p LEFT JOIN second as st ON st.site_id=p.site_id group by p.id ORDER BY p.id DESC LIMIT 10;

Answer 2

返回4列因为您的表格有重复site_id。

使用此更改您的查询。这将只返回2行。

SELECT `p`.* FROM `first` `p` LEFT JOIN `second` as `st` ON `st`.`site_id`=`p`.`site_id` GROUP BY p.id ORDER BY `id` DESC LIMIT 10;

Answer 3

它返回4行，因为两个表中的所有site_id都是11.要使它成为两列，只需在sql语句中添加Group By。

就像：

SELECT `p`.* FROM `first` `p` LEFT JOIN `second` as `st` ON 
`st`.`site_id`=`p`.`site_id` GROUP BY `p`.`id` ORDER BY `id` DESC LIMIT 10;

Answer 4

你问题的答案＆＃34;为什么＆＃34;是：这是正常的，因为您加入site_id并且第一个表中的每一行在第二个表中都有2个匹配的site_id's行，因此2 * 2 = 4

#first table
+-----+---------+
| id  | site_id |
+-----+---------+
| 31  |   11    |
+-----+---------+
| 32  |   11    |
+-----+---------+
# second table
+-----+---------+
| id  | site_id |
+-----+---------+
| 5   |   11    |
+-----+---------+
| 6   |   11    |
+-----+---------+

当您从{1}}的第一个表格加到第二个表格时，您将获得2行site_id

当您从{1}}的第一个表到第二个表加入32时，您将获得2行(31 with 5, and 31 with 6)

你需要将结果分组给@Sonu Bamniya说

在mysql查询中加入数据重复后

4 个答案: