use standard values if there is no overlap in table columns - mysql

Question

I have a table ce_relations and a table ce_values which i want to combine to a table ce_combined_values. The ce_combined_values table should have the exact same amount of rows as ce_relations. The query stated below does however only return the rows where the user_id and friend_id is existing in the ce_values.user_id column. I tried to solve this problem by using the IFNULL statement, but i guess there is also an extra condition missing in the WHERE clausule... any help is welcome!
INSERT INTO ce_combined_values (user_id, friend_id, relation_degree, user_value, friend_value, relation_value)
SELECT a.user_id, a.friend_id, a.relation_degree, IFNULL(b.1d_value, 0) as user_value, IFNULL(c.1d_value, 0) as friend_value, Least(b.1d_value, c.1d_value) as relation_value
FROM ce_relations a, ce_values b, ce_values c
WHERE a.relation_degree = 1 AND b.user_id = a.user_id AND c.user_id = a.friend_id AND b.user_id <> c.user_id
Union all
//same select query is used for relation_degree 2 with 2d_values and relation_degree 3 with 3d_values.
EDIT:
For example this is what I want to achieve:
table ce_relations:
+---------+-----------+-----------------+
| user_id | friend_id | relation_degree |
+---------+-----------+-----------------+
| 1 | 3 | 1 |
| 2 | 1 | 1 |
| 3 | 4 | 1 |
+---------+-----------+-----------------+
table ce_values:
+---------+----------+----------+----------+
| user_id | 1d_value | 2d_value | 3d_value |
+---------+----------+----------+----------+
| 1 | 5 | 10 | 33 |
| 2 | 10 | 12 | 44 |
| 3 | 20 | 13 | 55 |
+---------+----------+----------+----------+
should become ce_combined values ( deleted relation_degree and relation_value for readability)
+---------+-----------+------------+--------------+
| user_id | friend_id | user_value | friend_value |
+---------+-----------+------------+--------------+
| 1 | 3 | 5 | 20 |
| 2 | 1 | 10 | 5 |
| 3 | 4 | 20 | 0 |
+---------+-----------+------------+--------------+
but currently returns ( the row with friend_id = 4 is missing because it doesn't exist in ce_values)
+---------+-----------+------------+--------------+
| user_id | friend_id | user_value | friend_value |
+---------+-----------+------------+--------------+
| 1 | 3 | 5 | 20 |
| 2 | 1 | 10 | 5 |
+---------+-----------+------------+--------------+

Answer 1

If I understand your problem correctly, you need to LEFT JOIN your tables. And I don't think you need to UNION ALL the same query three times with just a different filter condition.
Maybe that helps you:
SELECT a.user_id, a.friend_id, a.relation_degree,
IFNULL(b.value, 0) as user_value,
IFNULL(c.value, 0) as friend_value,
Least(b.value, c.value) as relation_value
FROM ce_relations a
LEFT
JOIN ce_values b
ON a.user_id = b.user_id
LEFT
JOIN ce_values c
ON a.friend_id = c.user_id
AND b.user_id <> c.user_id
WHERE a.relation_degree IN (1, 2, 3)