Mysql - 从表 A 返回所有结果并选择性地与表 B 连接

Question

I have two tables:

default:

+----+--------+
| id | colour |
+----+--------+
| 1  | red    |
| 2  | green  |
| 3  | yellow |
+----+--------+

custom:

+--------+--------------+---------+
| linkId | customcolour | ownerId |
+--------+--------------+---------+
| 1      | bright red   | 1       |
| 2      | garden green | 2       |
+--------+--------------+---------+

I want to return everything from the default table and then get any associated customcolours (via the linkId). The query I am using is:

SELECT a.colour, b.customcolour 
FROM default a 
LEFT JOIN custom b ON a.id = b.linkId
WHERE (b.ownerId IS NULL OR b.ownerId = 1) 
GROUP BY a.id ORDER BY a.colur

However when I join to the custom table it will not select custom linkId 2 because the ownerId is not 1 nor NULL. Is there a way to return the row default.id = 2 and just set customercolour as NULL, without adding it to the table?

Answer 1

You need to move your WHERE criteria into the join

SELECT a.colour, b.customcolour 
FROM default a 
LEFT JOIN custom b ON a.id = b.linkId AND (b.ownerId IS NULL OR b.ownerId = 1)
GROUP BY a.id ORDER BY a.colur