How to join table to itself and select max values in SQL

Question

I have a contracts table:

contractId    date       price     partId
1             20120121   10        1
2             20110130   9         1
3             20130101   15        2
4             20110101   20        2

The contract with greatest date being the active contract (don't blame me, I blame infor for creating xpps)

I need to create query to see only active contracts (one contract per part, the contract with highest date).

So the result of the query should be like this:

contractId    date       price     partId
1             20120121   10        1
3             20130101   15        2

I am out of ideas here, I tried self joining the table, I tried aggregation functions, but I can't figure it out. If anyone would have any idea, please share them with me..

Answer 1

this will work on almost all RDBMs,

SELECT  a.*
FROM    tableName A
        INNER JOIN
        (
            SELECT partID, MAX(date) maxDate
            FROM    tableName
            GROUP BY partID
        ) B on a.partID = b.partID AND
                a.date = b.maxDate

• SQLFiddle Demo

if your RDBMS supports Window Function,

SELECT  contractId ,date, price,partId
FROM    
(
    SELECT contractId ,date, price,partId,
            ROW_NUMBER() OVER (PARTITION BY PartID
                                ORDER BY date DESC) rn
    FROM    tableName
) s
WHERE   rn = 1

• SQLFiddle Demo