SQL新手 - 子查询

Question

大家好，这是我的第一个问题，所以对我很轻松 - 需要一些帮助来提取一些数据。

这是我正在使用的两个表： 查询表：

+---------------------------------------------------------------------+
| ID (Primary Key) | Author | threadid | created     | Comments       |
+---------------------------------------------------------------------+
| 1                |   C    | 237      | 2016-07-24  | Hi there...    |
| 2                |   T    | 421      | 2015-06-07  | Hello, ..      |
| 3                |   C    | 421      | 2015-06-08  | Hi,...         |
| 4                |   C    | 327      | 2017-03-13  | Hey there..    |
+---------------------------------------------------------------------+

如果T代表公司向客户发送询问，则C代表客户向公司发送询问。

Enquirythreads表：

+----------------------------------+
| ID (Primary Key) |  created      |
+----------------------------------+
| 421              | 2016-07-24    |
| 237              | 2016-07-24    |
| 327              | 2015-06-08    |
+----------------------------------+

我想要的输出是：

+---------+
| ID      |
+---------+
|  421    |
+---------+

我想要所有 enquirythread id ，以便与作者T 建立与之关联的第一次查询。

这是我的代码，但无效：

SELECT enquirythreads.id
FROM enquirythreads
JOIN enquiries on enquirythreads.id = enquiries.threadid
WHERE enquiries.threadid IN 
    ( SELECT enquiries.threadid as enqid
      FROM 
            ( SELECT enquiries.threadid, min(enquiries.created) as mincreated
            FROM enquiries
            WHERE enquiries.author = 'T'
            GROUP BY enquiries.threadid ) x

     )

Answer 1

一种方法使用聚合和select e.threadid from enquiries e group by e.threadid having min(e.created) = min(case when e.author = 'T' then e.created end) ：

created

这表示：“检查最早的'T'日期是否与where的最早日期相同。”

另一种方法在select et.threadid from enquirythreads et where (select e2.author from enquiries e2 where e2.threadid = et.threadid order by e2.created asc fetch first 1 row only ) = 'T'; 子句中使用相关子查询：

{{1}}