Question

任何人都不知道如何选择它。表消息如下所示：

+----+------------+----------+---------+--------+-------------------+
| id | fromUserID | toUserID | message | status |         dt        |
+----+------------+----------+---------+--------+-------------------+ 
|  1 |      1     |     2    |Hey,wh...|  seen  |2018-08-15 17:43:51|
|  2 |      2     |     1    |   Hey   |  send  |2018-08-15 17:45:51|
+----+------------+----------+---------+--------+-------------------+

我要使用MySql查询：

SELECT m.id, 
       (SELECT m.message, IF(m.fromUserID='1',1,0), m.status, m.dt 
        FROM messages m 
        WHERE (m.fromUserID='1' OR m.toUserID='1') AND (m.fromUserID=u.id OR m.toUserID=u.id) 
        ORDER BY m.dt DESC 
        LIMIT 1) AS m,
       u.username,
       u.id AS usID
FROM messages m 
JOIN users u 
    ON u.id=IF(m.fromUserID='1',m.toUserID,m.fromUserID) 
WHERE (m.fromUserID='1' OR m.toUserID='1') AND
      m.dt>IFNULL((SELECT d.dt FROM deletechat d WHERE d.userID='1' AND d.withUserID=IF(m.fromUserID='1',m.toUserID,m.fromUserID)),'0000-00-00 00:00:00') 
GROUP BY u.id 
ORDER BY m.dt

我用了这个，但是我不能按日期排序。另外，如果您在JSON字符串中找到一些“，”，\'，\“，则我无法在PHP中读取它

SELECT m.id, 
       (SELECT CONCAT('{"message":"', m.message, '","myLast":"', IF(m.fromUserID='1',1,0), '", "status":"',m.status,'"}') 
        FROM messages m 
        WHERE (m.fromUserID='1' OR m.toUserID='1') AND 
              (m.fromUserID=u.id OR m.toUserID=u.id) 
        ORDER BY m.dt DESC 
        LIMIT 1) AS m,
       u.username,
       u.id AS usID 
FROM messages m 
JOIN users u 
    ON u.id=IF(m.fromUserID='1',m.toUserID,m.fromUserID)
WHERE (m.fromUserID='1' OR m.toUserID='1') AND
       m.dt>IFNULL((SELECT d.dt FROM deletechat d WHERE d.userID='1' AND d.withUserID=IF(m.fromUserID='1',m.toUserID,m.fromUserID)),'0000-00-00 00:00:00')
GROUP BY u.id

Answer 1

问题：我无法按日期排序

除了使用GROUP BY之外，您还可以使用另一个SELECT从当前查询中进行选择：

SELECT r.id, COALESCE(foo.url, bar.url) as url
FROM results r
LEFT JOIN foo USING (url_id)
LEFT JOIN bar USING (url_id)
WHERE foo.url IS NOT NULL OR bar.url IS NOT NULL;

问题：如果您在JSON字符串中找到一些'，'，\'，\“，则无法在PHP中读取

在较新的MYSQL版本中，该功能具有JSON函数，例如JSON_OBJECT() or JSON_QUOTE()。像这样使用：

SELECT * FROM 
   (SELECT m.id ...          /* <-- Your whole query */
   ...
   ORDER BY m.dt DESC        /* <-- At the end instead of GROUP BY user ORDER BY */
) as r LIMIT 1               /* <-- Select only the first result, which is the youngest */

对于较早的MySQL版本，您可以使用REPLACE()来转义所有引号等特殊字符。

在MySQL子查询和ORDER BY子查询的日期时间中选择多个列

1 个答案: