Question

I am using mysql and I have three tables T1, T2, T3
they contain data as follows:

+-------+
| t1.id |
+-------+
| 1    |
| 2    |
| 3    |
| 4    |
| 5    |
+------+

+-------+
| t2.id |
+-------+
| 1    |
| 2    |
| 3    |
| 4    |
+-------+

+-------+
| t3.id |
+-------+
| 1    |
| 2    |
| 3    |
+-------+

    I want to be able to select data as follows:

id      t1.id   t2.id   t3.id
1       1       1       1
2       2       2       2
3       3       3       3
4       4       4       null
5       5       null    null

How can I do this?

Answer 1

SELECT t1.id, t2.id, t3.id
FROM t1 
LEFT JOIN t2 ON t1.id=t2.id
LEFT JOIN t3 ON t1.id=t3.id

works, if t1 is guaranteed to have a strict superset of all ids.

If not, you would need a helper table:

SELECT th.id,t1.id, t2.id, t3.id
FROM (
  SELECT id FROM t1
  UNION DISTINCT 
  SELECT id FROM t2
  UNION DISTINCT 
  SELECT id FROM t3
) AS th
LEFT JOIN t1 ON th.id=t1.id
LEFT JOIN t2 ON th.id=t2.id
LEFT JOIN t3 ON th.id=t3.id