在 SQL 中计算带有 where 子句的字段

Posted 2023-02-24

【中文标题】在 SQL 中计算带有 where 子句的字段

【英文标题】：Count fields with a where clause in SQL

【发布时间】：2011-10-25 04:19:58

【问题描述】：

是否可以计算某些内容的字段？

我有以下查询：

SELECT * FROM 
(SELECT messages.id as id,
       messages.user_id,
       messages.category_id,
       messages.parent_id,
       messages.message,
       messages.create_date,
       messages.update_date,
       messages.status,
       users.name as username,
       users.id as userid,
       users.email as useremail,
       users.phone as userphone,
       users.active as useractive,
       users.role as userroel,
       users.date as register_date
FROM `afo_messages` as messages 
INNER JOIN `key_users` as users ON messages.user_id = users.id
WHERE messages.category_id=5 AND messages.parent_id=0
ORDER BY messages.id DESC LIMIT 30) ilv 
ORDER BY id ASC

（这里是输出示例：http://lab.mirgorod.us/res.pdf）

是否可以在此查询中再添加一列：子消息计数（parent_id = 父消息的 id）？

【参考方案1】：

您可以使用子查询轻松完成：

SELECT * FROM 
(SELECT messages.id as id,
       messages.user_id,
       messages.category_id,
       messages.parent_id,
       messages.message,
       messages.create_date,
       messages.update_date,
       messages.status,
       users.name as username,
       users.id as userid,
       users.email as useremail,
       users.phone as userphone,
       users.active as useractive,
       users.role as userroel,
       users.date as register_date,
       (SELECT COUNT(*) FROM afo_messages WHERE parent_id=messages.id) AS `sub messages`
FROM `afo_messages` as messages 
INNER JOIN `key_users` as users ON messages.user_id = users.id
WHERE messages.category_id=5 AND messages.parent_id=0
ORDER BY messages.id DESC LIMIT 30) ilv 
ORDER BY id ASC

【讨论】：

但是如何处理我的查询呢？

只需添加以 (SELECT COUNT... 开头的行，仅此而已。它就像另一个字段。如果你想要我可以发布整个查询，我会缩短它。

【参考方案2】：

尝试将其添加为列：

child_count = (select count(*) from afo_messages where parent_id=messages.id)