mysql - 连接2个mysql表时如何仅获取第二个表中所需的相关行

Question

我在MySQL数据库中有以下2个表A和B。当我通过 VisitID (A.VisitID = B.VisitID) 连接 2 个表时，我只想从表 B 中提取与 VisitID 相关的最新操作日期和时间记录。例如，对于 VisitID = 85，我想选取表 B 中具有 15/10/2014 3:44 的行。对于 VisitID = 86，我只想选取表 B 中具有 09/10/2014 1:28 的行。

表A:

VisitID       VisitTitle                     VisitSummary                        Conclusion

85            Go to Paddy Field 1            Checked Temperatures                1
86            Soil Quality Checked           Checked PHP of different soil       2
87            Go to Paddy Field 2            Collected Soil samples              0

表B:

RefID   VisitID      ActionDesc                                 Actiondateandtime

1       85           Submiited to Management                    9/10/2014  12:03
2       86           Sent to lab                                9/10/2014  1:06
3       86           Sent to lab                                9/10/2014  1:07
4       86           Sent to lab                                9/10/2014  1:21
5       86           Sent to lab                                9/10/2014  1:28
6       87           Followed with Soil scientist               9/10/2014  1:32
7       87           Followed with Soil scientist               9/10/2014  1:33
8       85           Submitted to Management                    15/10/2014 3:44

我想要这两个表连接后的结果

A.VisitID  A.VisitTitle           A.Conclusion  B.RefID   B.VisitID  B.Actiondateandtime

85         Go to Paddy Field 1    1             8         85         15/10/2014 3:44
86         Soil Quality Checked   2             5         86         9/10/2014 1:28
87         Go to Paddy Field 2    0             7         87         9/10/2014 1:33

需要什么 MySQL 代码才能获得所需的结果？

Answer 1

一种选择是使用 max 聚合将表加入自身，以获取按 accessid 分组的最大日期:

select a.visitid, a.visittitle, a.conclusion, b.refid, b.actiondateandtime
from tablea a 
  join tableb b on a.visitid = b.visitid
  join (
    select visitid, max(actiondateandtime) actiondateandtime
    from tableb 
    group by visitid
    ) c on b.visitid = c.visitid and b.actiondateandtime = c.actiondateandtime

• SQL Fiddle Demo