Question

我使用以下MySQL查询来访问聊天记录中的最新20条消息并反转订单，以便最后在屏幕上打印最新消息：

SELECT * 
FROM 
    (
        SELECT
            messageID,
            posterID,
            messageTime,
            message
        FROM
            chat_messages
        /* Subquery is used to get the most recent 20 by messageTime */
        ORDER BY 
            messageTime DESC
        LIMIT 20
    ) subq
/* Reorder the result of the subquery to put them back into ascending order */
ORDER BY 
    messageTime ASC

效果很好。问题是我现在正在尝试向聊天功能添加组。在这样做的过程中，我在名为“group”的chat_messages表中添加了另一列。主聊天日志是组0，因此我需要更改上述查询以仅访问主聊天日志中的消息。这是我被困的地方。似乎MySQL不允许我在子查询中添加where子句。我尝试过以下操作但没有用：

SELECT * 
FROM 
    (
        SELECT
            messageID,
            posterID,
            messageTime,
            message
        FROM
            chat_messages
        WHERE
            group = '0'
        /* Subquery is used to get the most recent 20 by messageTime */
        ORDER BY 
            messageTime DESC
        LIMIT 20
    ) subq
/* Reorder the result of the subquery to put them back into ascending order */
ORDER BY 
    messageTime ASC

我收到此错误消息（第58行是查询后的下一行）：

警告：mysql_num_rows（）期望参数1为资源，第58行的xxxxxxx中给出布尔值

按照this thread上的内容，我尝试了以下操作，但也没有用：

SELECT * 
FROM 
    (
        SELECT
                (
                    SELECT
                        messageID,
                        posterID,
                        messageTime,
                        message
                    FROM
                        chat_messages
                    WHERE
                        group = '0'
                )
        FROM
            chat_messages
        /* Subquery is used to get the most recent 20 by messageTime */
        ORDER BY 
            messageTime DESC
        LIMIT 20
    ) subq
/* Reorder the result of the subquery to put them back into ascending order */
ORDER BY 
    messageTime ASC

如何使查询仅访问组0中的消息？我只是不明白它是如何起作用的？

谢谢你，
乔

Answer 1

GROUP是MySQL的reserved word，在其周围加上`。

SELECT * 
FROM 
(
     SELECT messageID,
          posterID,
          messageTime,
          message
     FROM chat_messages
     WHERE `group` = '0'
        /* Subquery is used to get the most recent 20 by messageTime */
     ORDER BY  messageTime DESC
     LIMIT 20
 ) subq
/* Reorder the result of the subquery to put them back into ascending order */
ORDER BY  messageTime ASC

Answer 2

SELECT messageID, posterId, messageTime, message FROM chat WHERE `group` = [number] ORDER BY messageTIME DESC LIMIT 20

这将为您提供最新的20条消息。正如@bluefeet所说，GROUP是一个保留字，你应该使用反引号。

如何在MySQL子查询中使用where子句？

2 个答案: