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予期しない空白行を返すように見えるSQLクエリがあります。このクエリには別のテーブルへの右結合がありますが、行を返すとnull値が返されます。

  SELECT e.error_id, e.user_id, e.error_title, e.error_content, e.error_answers, 
         e.error_votes, e.error_views, e.error_added_date, u.user_name
  FROM errors e
  JOIN users u
    ON u.user_id=e.user_id
  RIGHT JOIN answers a
    ON a.error_id=e.error_id AND a.answer_accepted='1'
  GROUP BY e.error_id
  ORDER BY e.error_added_date DESC

このクエリは1つの行を返すはずですが、期待される行と空白の値を持つ1つの行が返されます。それはなぜですか。

エントリー

+----------------------------------------------------------------------------------------------------+
answer_id | error_id | user_id | answer_content | answer_accepted | answer_votes | answer_added_date |
1         | 3        | 1       | text           | 0               | 0            | 2013-01-31 12:49:12
2         | 3        | 1       | text           | 1               | 1            | 2013-01-31 12:52:29
3         | 3        | 1       | text           | 0               |-1            | 2013-01-31 12:53:45
4         | 2        | 1       | text           | 0               |-1            | 2013-01-31 12:53:45
+----------------------------------------------------------------------------------------------------+

結果：

+-------------------------------------------------------------------------------+
|   1 | 1    | text |  3   | 0    |  2   |  2013-01-29 16:56:20  |  Mihai Matei |
|NULL | NULL | NULL | NULL | NULL | NULL |       NULL            |  NULL        |
+-------------------------------------------------------------------------------+
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1 に答える 1

2

This is happening because you are doing a Right Join. I think you want a Left Join or an Inner Join.

A right join means return all the table rows on the right and the matching rows from the left. Because your result set doesn't include any columns from the answers table (the right table), you can get a set of all nulls. In other words, there is a row in the answers table with no corresponding rows in the error and users table.

Given the additional criteria in the comment on this answer, here's the query I would try:

  SELECT e.error_id, e.user_id, e.error_title, e.error_content, e.error_answers, 
         e.error_votes, e.error_views, e.error_added_date, u.user_name
  FROM errors e
  JOIN users u
    ON u.user_id=e.user_id
  LEFT JOIN answers a
    ON a.error_id = e.error_id and a.answer_accepted = '1'
  WHERE a.answer_id is null
  GROUP BY e.error_id
  ORDER BY e.error_added_date DESC
于 2013-01-31T20:08:03.977 に答える