在MySQL中用不相等的字段联接multible表

Question

I have tried some time now. I tried UNION and fake (one left and one right join) full join's. Im not that good with sql so maybe i stumbled over the solution but couldn't do it right.

For this example i have three tables (i probably have more with the time).

post:
 ----------------------------
|  type  |  rel_id  |   id   |
 ----------------------------
|  video |     2    |    51  |
|----------------------------|
|  text  |     1    |    50  |
 ----------------------------

video:
 ----------------------------
|  id |  video_url |  title  |
 ----------------------------
|  2  |     ...    |    ...  |
|----------------------------|
|  1  |     ...    |    ...  |
 ----------------------------

text:
 ----------------
|  id |  contend |
 ----------------
|  2  |    ...   |
|----------------|
|  1  |    ...   |
 ----------------

The post table is the main table. The type field shows what table to join with. the field rel_id says what row to connect to.

I could use some output like this:

 ------------------------------------------
| id | type  | video_url | title | contend |
 ------------------------------------------
| 51 | video |    ...    |  ...  |   NULL  |
|------------------------------------------|
| 50 | text  |    NULL   |  NULL |   ...   |
 ------------------------------------------

The last problem is that the last WHERE clause need to be free so that i can use it for searching all results.

Answer 1

You could try:

SELECT p.id AS id, p.type AS type, v.video_url AS video_url, 
       v.title AS title, NULL AS contend
FROM post p INNER JOIN video v 
  ON p.rel_id = v.id AND p.type = 'video'
UNION
SELECT p.id, p.type, NULL, NULL, t.contend
FROM post p INNER JOIN text t
  ON p.rel_id = t.id AND p.type = 'text'