In addition to the much needed unique constraint, various other index-based constraints and "mutators" (for the lack of a better term) would be helpful.

Ideas for Constraints:

unique == field on inserted element must be unique
gt == field on inserted element must be greater than current MAX_VALUE
lt == field on inserted element must be smaller than current MIN_VALUE

"Mutators" would be things like:

auto_increment == field on inserted element auto increments to MAX_VALUE+1
trigger == provided javascript is executed on this field of all inserted elements

Feel free to edit this text if there's a better, established term for what I called "mutators" here.