左联接不返回值

Question

This query is giving values as null for accessories_type.type but this table has values:

SELECT accessories.id, accessories.name, accessories_type.type
FROM accessories
LEFT JOIN accessories_type ON accessories_type.type = accessories.accessories_type_id;

Answer 1

Your query is a little strange. The value accessories_type.type in the select clause is the same as being used for the join.

Assuming there are no duplicate rows in the type table, you can get the same effect by doing:

select id, name, accessories_type_id
from accessories

Your queries is additionally looking for matches in the second table. When there are no matches, you are getting NULL.

Normally, I would expect something like accessories_type.typename in the select . This gives me an idea. Perhaps you really mean:

SELECT accessories.id, accessories.name, accessories_type.type
FROM accessories
LEFT JOIN accessories_type ON accessories_type.id = accessories.accessories_type_id;

That is, you should be joining on the id . You are joining on the name and there is no match (in general).