only select row with later expiry date

if I have a table like the following, how could I only select out those serial_number and its contract_type with later expiry date?

serial_number      contract_type         expiry_date
abc001                  SPRT              2011-05-31 00:00:00
abc001                  HOMD              2013-05-31 00:00:00
abc002                  SPRT              2012-10-14 00:00:00
abc002                  HOMD 开发者_如何学编程             2011-10-14 00:00:00
abc003                  SPRT              2014-05-31 00:00:00
abc003                  HOMD              2011-05-31 00:00:00
................

1) I could make the assumption if it makes this query simpler: each serial_number(SN) will have two and only two contract_types in the table.

2) the actual situation is: SN and contract_type are the primary key, and I'm only looking for the contract_type 'SPRT' and 'HOMD'.

The final result set I need is:

• SN with only 'SPRT' or 'HOMD' contract_type

• if SN has both 'SPRT' and 'HOMD', I only need the SN's record with later expiry date (if they have the same expiry date, only pick one)

Anyone could give out the query? the actual case might be too complicated to get in one query, but how about the first simplified case.

SELECT t.serial_number, t.contract_type, t.expiry_date
    FROM YourTable t
        INNER JOIN (SELECT serial_number, MAX(expiry_date) AS MaxDate
                        FROM YourTable
                        WHERE contract_type IN ('SPRT', 'HOMD')
                        GROUP BY serial_number) q
            ON t.serial_number = q.serial_number
                AND t.expiry_date = q.MaxDate
    WHERE t.contract_type IN ('SPRT', 'HOMD')