Just make another field in table having name IsVacant and having datatype int or bit
specify if room is alloted then the IsVacant 1 else it is 0
Then you have to just code in sql like that for dropdown
select * from TableName where IsVacant=0
this query returns list of vacant room only..
You can count also with group by clause
like that.. :)
select count(*) from TableName group by IsVacant
It gives you count of alloted as well Vacant rooms