Question

这是我目前的数据库结构：

// users
+----+--------+
| id |  name  |
+----+--------+
| 1  | Jack   |
| 2  | Peter  |
| 3  | John   |
| 4  | Barman |
| 5  | Ali    |
+----+--------+

// vote
+----------+---------------+
| voter_id | owner_post_id |
+----------+---------------+
| 2        | 3             |
| 4        | 2             |
| 1        | 1             | -- people can vote to its own posts
| 5        | 2             |
| 2        | 2             |
| 3        | 2             |
+----------+---------------+

这是预期的输出：（对于用户2）

+---------+-----------+----------+------------+
| user_id | user_name | voter_id | voter_name |
+---------+-----------+----------+------------+
| 2       | Peter     | 4        | Barman     |
| 2       | Peter     | 5        | Ali        |
| 2       | Peter     | 3        | John       |
+---------+-----------+----------+------------+

注意：通常结果中应该还有一行，如下所示：

| 2       | Peter     | 2        | Peter      |

但我不想得到user_id和voter_id相同的那些。我怎么能这样做？

我当前的查询：

SELECT u.id, u.name, v.voter_id
FROM users u
INNER JOIN vote v
ON u.id = v.owner_post_id
WHERE u.id = 2 -- for user 2

Answer 1

你需要加入users表两次，一次为投票关系的每一方。

SELECT t2.id AS user_id,
       t2.name AS user_name,
       t3.id AS voter_id,
       t3.name AS voter_name
FROM vote t1
INNER JOIN users t2
    ON t1.owner_post_id = t2.id
INNER JOIN users t3
    ON t1.voter_id = t3.id
WHERE t1.owner_post_id = 2 AND
      t1.voter_id <> t1.owner_post_id

如何从用户表中获取选民姓名？

1 个答案: