用 IF 语句查询答案

【问题标题】：Query with IF statement用 IF 语句查询
【发布时间】：2012-02-03 08:10:56
【问题描述】：

我有一个关于 MYSQL If 语句的问题。我的要求：

SELECT c.status, c.thumb, j.id, j.name, j.username, s.session_id, a.time, a.isactive, a.country, a.countrycode, a.city
FROM j16_users AS j 
JOIN j16_community_users AS c ON c.userid = j.id 
LEFT JOIN j16_session AS s ON s.userid = j.id 
LEFT JOIN j16_session AS s ON s.userid = j.id 
LEFT JOIN j16_x5_fastchat_users AS a ON a.userid = j.id 
WHERE j.id IN (62,66,2692)

我怎样才能添加一个新列：isonline with condition to above request:

IF(a.time > DATE_SUB(NOW(), INTERVAL 1 MINUTE), "1", "0") AS isonline

谢谢！

【问题讨论】：

标签： mysql select if-statement

【解决方案1】：

SELECT c.status, c.thumb, 
       j.id, j.name, 
       j.username, s.session_id, 
       a.time, a.isactive, 
       a.country, a.countrycode, a.city,
       IF(a.time > DATE_SUB(NOW(), INTERVAL 1 MINUTE), 1, 0) AS isOnline
FROM j16_users AS j 
     JOIN j16_community_users AS c ON c.userid = j.id 
     LEFT JOIN j16_session AS s ON s.userid = j.id 
     LEFT JOIN j16_session AS s ON s.userid = j.id 
     LEFT JOIN j16_x5_fastchat_users AS a ON a.userid = j.id 
WHERE j.id IN (62,66,2692)

【讨论】：

【解决方案2】：

只需将其添加到SELECT 列表中即可。不过建议使用CASE statement，因为它更便携。

SELECT 
  c.status, 
  c.thumb, 
  j.id, 
  j.name, 
  j.username, 
  s.session_id, 
  a.time, 
  a.isactive, 
  a.country, 
  a.countrycode, 
  a.city,
  CASE WHEN a.time > DATE_SUB(NOW(), INTERVAL 1 MINUTE) THEN 1 ELSE 0 END AS isonline
FROM
  j16_users AS j 
  JOIN j16_community_users AS c ON c.userid = j.id 
  LEFT JOIN j16_session AS s ON s.userid = j.id 
  LEFT JOIN j16_session AS s ON s.userid = j.id 
  LEFT JOIN j16_x5_fastchat_users AS a ON a.userid = j.id 
WHERE 
  j.id IN (62,66,2692)

由于您只需要布尔值 1 或 0，因此该语句也可以不使用 CASE 或 IF 编写为：

 ... 
 a.countrycode, 
 a.city,
 (a.time > DATE_SUB(NOW(), INTERVAL 1 MINUTE)) AS isonline
 ...

也会有同样的效果。

【讨论】：