I have the need to record quantities that would never be more or less than a half. Normal data would be an integer value, but in some cases I may have to record a .5 value. Is there a way I can force mySql to accept only values evenly divided by .5? I'll do this checking in my application too, but I want to ensure the data in the table is always correct.
If some future version of MySQL supported CHECK CONSTRAINTS, then you could use one of them and store the data as you'd actually use it. Be wary, because MySQL will happily parse check constraints, but it does not currently enforce them.
For all present versions of MySQL, I think that r937's suggestion is as good as you can get.