子查询中的限制

Question

我有一个可以包含主要消息的消息表，以及子消息。我想要实现的只是获得前2个最新的子消息。

SELECT message.id, message.date_sent,message.object_id, message.content, 
       SubMessage1.content, SubMessage1.id, SubMessage1.date_sent,
       SubMessage2.content, SubMessage2.id, SubMessage2.date_sent
  FROM messages As Message 
  LEFT
 OUTER
  JOIN ( SELECT messages.object_id as object_id, messages.content as content,
                messages.id as id , messages.date_sent as date_sent
           FROM messages
          ORDER
             BY messages.date_sent ASC LIMIT 1,1
       ) as SubMessage1
    ON Message.id = SubMessage1.object_id
  LEFT
 OUTER
  JOIN ( SELECT messages.object_id as object_id, messages.content as content,
                messages.id as id , messages.date_sent as date_sent
           FROM messages
          ORDER
             BY messages.date_sent ASC LIMIT 2,1
       ) as SubMessage2
    ON Message.id = SubMessage2.object_id
 WHERE Message.id = 1

当我删除限制时，我总是得到第一个结果。但是当我加入限制时，我得到空值。有任何想法吗？建议？

Answer 1

SELECT * FROM messages WHERE id = 1
Union all
SELECT * FROM messages WHERE object_id=1
ORDER BY date_sent ASC LIMIT 2

Answer 2

这里的问题是子节点中的ORDER BY ... LIMIT ...在连接中的ON Message.id = ... .object_id之前被应用。因此，联接只有机会检查一个消息并确定它是否是子消息。大多数情况下，它不会，因此您将获得NULL值。

要解决此问题，您需要将ON - 子句的内容移动到子查询中的WHERE - 子句中。通常这会有点混乱，但由于你只是想要检索单记录结果集，所以它并不太糟糕：

SELECT message.id, message.date_sent, message.object_id, message.content, 
       SubMessage1.content, SubMessage1.id, SubMessage1.date_sent,
       SubMessage2.content, SubMessage2.id, SubMessage2.date_sent
  FROM messages AS Message 
  LEFT
 OUTER
  JOIN ( SELECT messages.object_id AS object_id, messages.content AS content,
                messages.id AS id, messages.date_sent AS date_sent
           FROM messages
          WHERE messages.object_id = 1
          ORDER
             BY messages.date_sent ASC LIMIT 1,1
       ) AS SubMessage1
    ON TRUE
  LEFT
 OUTER
  JOIN ( SELECT messages.object_id AS object_id, messages.content AS content,
                messages.id AS id, messages.date_sent AS date_sent
           FROM messages
          WHERE messages.object_id = 1
          ORDER
             BY messages.date_sent ASC LIMIT 2,1
       ) AS SubMessage2
    ON TRUE
 WHERE Message.id = 1
;

（请注意，这在查询中包含三次参数：可能不太理想，但实际上不是问题。）

Answer 3

如果您想要2行，请使用

LIMIT 2

在SQL领域，计数从1而不是0。

另外，在mysql LIMIT中有2个参数：

LIMIT offset, row_count

但是有1个参数：

LIMIT row_count

我想见到这样的天才，他决定像那样移动参数并踢他的裤子。

Answer 4

LIMIT命令的起始索引为0.您可能希望将限制更改为（0,1）和（1,1）。