SQL在另一个表中匹配ID并加入

Question

Id fields in article , tweet are foreign keys to the posts.post_id .

In MySQL, I would like to query for example, post_id=2 .

How do i get the desired result from table posts + tweet (since id=2 only exists in tweet table)

post_id | author | created_at .... | tweet_id | message     | ... all cols from tweet
2       | B      | ...........     | 2        | Hello world | ...

When query post_id=1 , result will be col from posts + article

post_id | author | created_at.... | article_id | title | ... all cols from article
1       | A             ..............         | A     | ...

Thanks for helping.

---

Database fiddle: http://sqlfiddle.com/#!9/be4f302/21

posts

| post_id | author | created_at, modified_at....
|---------|--------|-----------
| 1       | A      | ...
| 2       | B      | ...
| 3       | C      | ...
| 4       | D      | ...
| 5       | E      | ...

article

| article_id | title | ...
|------------|-------|----
| 1          | A     | 
| 3          | B     | 

tweet

| tweet_id | message     | ...
|----------|-------------|---
| 2        | Hello World | 

Answer 1

If I understand this right, you might be after a LEFT JOIN and coalesce() .

SELECT p.*,
       coalesce(a.article_id, t.tweet_id) article_id_or_tweet_id,
       coalesce(a.title, t.message) article_title_or_tweet_message
       FROM posts p
            LEFT JOIN article a
                      ON a.article_id = p.post_id
            LEFT JOIN tweet t
                      ON t.tweet_id = p.post_id
       WHERE p.post_id = ?;

(Replace the ? with the ID of the post you want to query.)

Answer 2

Here is a query that you can try out..

SELECT P.*, A.title, T.message FROM posts P LEFT JOIN article A ON (P.post_id = A.article_id) LEFT JOIN tweet T ON (T.tweet_id = P.post_id)

Hope this helped you.

Answer 3

You should be able to use UNION since only one of the SELECT will return a row for a given id

SELECT p.*, a.title as 'post'
FROM posts p
JOIN article a ON p.post_id = a.article_id
WHERE p.post_id = 2
UNION 
SELECT p.*, m.message as 'post'
FROM posts p
JOIN tweet t ON p.post_id = t.tweet_id
WHERE p.post_id = 2