Question

我有3个表，它们有一个公共字段user_id

gain_table

+--------------------------+
|user_id    |   gain_count |
| 1         |   3          |
| 2         |   4          |
| 3         |   1          |
+--------------------------+

consum_table

+--------------------------+
|user_id    |consume_count |
| 2         |   5          |
| 5         |   4          |
| 6         |   7          |
+--------------------------+

join_table

+--------------------------+
|user_id    |   join_count |
| 1         |   3          |
| 2         |   1          |
| 5         |   4          |
+--------------------------+

我想要这样的输出：

输出：

+-----------+--------------+--------------+------------+
|user_id    |   gain_count |consume_count | join_count |
| 1         |   3          |     0        | 3          |
| 2         |   4          |     5        | 1          |
| 3         |   1          |     0        | 0          |
| 5         |   0          |     4        | 4          |
| 6         |   0          |     7        | 0          |
+-----------+--------------+--------------+------------+

是的，我希望将这三个表合并为一个表，如果某些字段值为空，则将该字段值赋予0。

如何编写MySQL查询？

Answer 1

您可以使用所有表中的UNION来获得所需的结果，为每个表中不存在的值选择0，然后按user_id对所有字段求和：

SELECT user_id, SUM(gain_count) AS gain_count, 
    SUM(consume_count) AS consume_count, SUM(join_count) AS join_count
FROM (SELECT user_id, gain_count, 0 AS consume_count, 0 AS join_count FROM gain_table
      UNION ALL
      SELECT user_id, 0, consume_count, 0 FROM consume_table
      UNION ALL
      SELECT user_id, 0, 0, join_count FROM join_table) u
GROUP BY user_id

输出：

user_id     gain_count  consume_count   join_count
1           3           0               3
2           4           5               1
3           1           0               0
5           0           4               4
6           0           7               0

Answer 2

以下是使用ansi标准sql的方法：

SELECT coalesce(g.user_id, c.user_id, j.user_id) user_id
    , coalesce(g.gain_count, 0) gain_count
    , coalesce(c.consume_count, 0) consume_count
    , coalesce(j.join_count,0) join_count
FROM gain_table g
FULL JOIN consume_table c on c.user_id = g.user_id
FULL JOIN join_table j on j.user_id = coalsece(c.user_id, g.user_id)

自92版本以来，这已成为ansi标准的一部分，除MySql以外的每个主要数据库都可以做到。 MySql必须这样做：

SELECT ids.user_id
    , coalesce(g.gain_count, 0) gain_count
    , coalesce(c.consume_count, 0) consume_count
    , coalesce(j.join_count, 0) join_count
FROM (
    SELECT user_id FROM gain_table
    UNION
    SELECT user_id FROM consume_table
    UNION
    SELECT user_id FROM join_table
) ids
LEFT JOIN gain_table g on g.user_id = ids.user_id
LEFT JOIN consume_table c on c.user_id = ids.user_id
LEFT JOIN join_table j on j.user_id = ids.user_id

MySQL三表查询或联合

2 个答案: