SQL Query - Grouping with multiple tables

问题

There are 2 tables involved in this question:

Table1:

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

Table 2:

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

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

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        -|

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.

If I do:

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

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).

I hope someone can provide some ideas, Thanks

EDIT:

Table 1 has a Unique ID column:

|- 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              -|

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

* whew *

回答1:

Ahhh, the good old greatest-n-per-group...

Here's one way to do it:

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

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

回答2:

You could make your query into a subquery (called states in my version) and then join that back into table1 again and grab the extra columns, like this:

SELECT * FROM

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

LEFT JOIN table1 t1_again
ON states.specialNumber = t1_again.specialNumber

来源：https://stackoverflow.com/questions/8170443/sql-query-grouping-with-multiple-tables

标签

sql

greatest-n-per-group