我不明白为什么这个问题是错误的。我单独尝试了真查询和假查询，结果都成功了，但是当我把它们放在if子句中时，却没有成功。提前感谢您的帮助：）

SELECT 
IF ( 
    (SELECT useronline FROM wp_livechat ORDER BY id DESC LIMIT 1) = 1
    , 
    (SELECT *
     FROM wp_livechat
     WHERE id >= (SELECT id 
                  FROM wp_livechat
                  WHERE useronline = 0
                  ORDER BY id DESC
                  LIMIT 0, 1)
     AND id <= (SELECT id 
                FROM wp_livechat
                WHERE useronline = 1
                ORDER BY id DESC
                LIMIT 0, 1)
    ),
    (SELECT *
     FROM wp_livechat
     WHERE id >= (SELECT id 
                  FROM wp_livechat
                  WHERE useronline = 0
                  ORDER BY id DESC
                  LIMIT 1, 1)
     AND id <= (SELECT id 
                FROM wp_livechat
                WHERE useronline = 0
                ORDER BY id DESC
                LIMIT 0, 1)
    )
);

编辑：这是表格：https://ibb.co/huri6e
我想获取从最后一个useronline=0到最后一个useronline=0的消息。如果最后一个条目是useronline=1，那么我想从useronline=1的最后一个条目到useronline=0的最后一个条目检索消息。