MySQL左联接未按预期工作

Question

Table Name : Feature

+--------+----------+
| fea_id | fea_name |
+--------+----------+
|      1 | Price    |
|      2 | Height   |
|      3 | Weight   |
+--------+----------+

Table Name : property_meta

+----+--------+--------+-------+
| id   | fea_id | pro_id | value |
+----+--------+--------+-------+
|  100 |      1 |    300 |  2500 |
|  200 |      2 |    300 |   300 |
|  
+----+--------+--------+-------+

My Query

SELECT * FROM feature LEFT JOIN property_meta ON feature.fea_id = property_meta.fea_id where property_meta.pro_id=300 GROUP by feature.fea_id ORDER BY feature.fea_id ASC

Expected Result

+--------+--------+-------+
| fea_id | pro_id | value |
+--------+--------+-------+
|      1 |    300 | 2500  |
|      2 |    300 | 300   |
|      3 |    300 | NULL  |
+--------+--------+-------+

But I am getting without the last row . I need that last row also . How I modify my query to get the last row also?

That means I need to get all rows of Feature table even there is no value in property meta table.

Answer 1

where property_meta.pro_id=300 makes your left join to an inner join . Add this to the on clause and it is working:

SELECT * FROM feature LEFT JOIN property_meta ON feature.fea_id = property_meta.fea_id and property_meta.pro_id=300 GROUP by feature.fea_id ORDER BY feature.fea_id ASC

Answer 2

SELECT * FROM feature LEFT JOIN property_meta ON feature.fea_id = property_meta.fea_id and property_meta.pro_id=300 GROUP by feature.fea_id ORDER BY feature.fea_id ASC

将where条件放入连接条件中，因为where条件限制了结果，而join条件仅连接了表

Answer 3

SELECT * FROM feature LEFT JOIN property_meta 

ON feature.fea_id = property_meta.fea_id 
   AND property_meta.pro_id=300 -- <-- need to move this condition here or the where clause will remove the last row

GROUP by feature.fea_id ORDER BY feature.fea_id ASC

Answer 4

where property_meta.pro_id=300 removes that line, cause this will be null after the unsuccessfull join.

remove that condition and you will have

+--------+--------+-------+
| fea_id | pro_id | value |
+--------+--------+-------+
|      1 |    300 | 2500  |
|      2 |    300 | 300   |
|      3 |    NULL| NULL  |
+--------+--------+-------+

Fix accordingly :)