SQL查询-与多个表分组 [英] SQL Query - Grouping with multiple tables

问题描述

此问题涉及2个表:

表1:

|- Time Stamp -|- Special Number -|- Other Data -|
|- 2011       -|- 1              -|- green      -|
|- 2010       -|- 1              -|- blue       -|
|- 2009       -|- 2              -|- yellow     -|
|- 2011       -|- 3              -|- red        -|
|- 2010       -|- 3              -|- orange     -|
|- 2009       -|- 4              -|- purple     -|

表2:

|- Special Number -|- State (location) -|
|- 1              -|- Hawaii           -|
|- 2              -|- Hawaii           -|
|- 3              -|- Alaska           -|
|- 4              -|- Alaska           -|

表2将特殊编号"与州"相关联

Table 2 relates the 'Special Number' to a 'State'

现在，我尝试获得的结果将类似于:

Now then, the result I am trying to get would look something like:

|- Time Stamp -|- State (location) -|- Other Data -|
|- 2011       -|- Hawaii           -|- green      -|
|- 2011       -|- Alaska           -|- red        -|

我要在哪里获取MAX时间戳(按每个州分组)以及表1中与该最新时间戳"行相对应的其他数据.

Where I am trying to get the MAX time stamp, grouped by each state, as well as the other data corresponding to that 'latest time stamp' row from Table 1.

如果我这样做:

SELECT MAX(time stamp), state
FROM table 1, table 2
WHERE table 1.special number = table 2.special number
GROUP BY state

这将返回每个状态的最大时间戳(这几乎是我想要的状态)，但是当我尝试包含其他数据"时，它将返回所有记录(因为每个其他数据"记录都是唯一的) ).

This returns the max time stamp, for each state (which is almost what I am looking for), but when I try to include 'Other Data', it returns all the records (since every 'other data' record is unique).

我希望有人能提供一些想法， 谢谢

I hope someone can provide some ideas, Thanks

表1的唯一ID"列:

|- Time Stamp -|- Special Number -|- Other Data -|- Unique Row ID -|
|- 2011       -|- 1              -|- green      -| 0              -|
|- 2010       -|- 1              -|- blue       -| 1              -|
|- 2009       -|- 2              -|- yellow     -| 2              -|
|- 2011       -|- 3              -|- red        -| 3              -|
|- 2010       -|- 3              -|- orange     -| 4              -|
|- 2009       -|- 4              -|- purple     -| 5              -|

解决方案* * * *感谢所有发布者！ * * * *

EDIT 2: SOLUTION * * * * Thanks to everyone who posted ! * * * *

 SELECT t1.timestamp, t2.specialNumber, t1.otherData
 FROM Table1 t1 inner join Table2 t2 on t1.specialNumber = t2.specialNumber
     inner join (select MAX(Table1.timestamp) maxts, Table2.state
         from Table1 inner join Table2 on Table1.specialNumber = Table2.specialNumber
         group by Table2.state) t3
     on t2.state = t3.state and t1.timestamp = t3.maxts

* * *

推荐答案

啊，好老的 greatest-n-per-group ...

这是一种方法:

SELECT t1.TimeStamp, t2.State, t1.OtherData
FROM Table1 t1
inner join Table2 t2 
    on t1.SpecialNumber = t2.SpecialNumber
inner join (SELECT MAX(time stamp) maxts, state
            FROM table1 inner join table2
            ON table1.specialnumber = table2.specialnumber
            GROUP BY state) t3
    on t2.State = t3.State and t1.TimeStamp = t3.maxts

这里有一个非常全面的答案:

There's a very comprehensive answer here: SQL Select only rows with Max Value on a Column

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