Mysql unique constraint allowing single row for a combination

Question

Is it possible to have a unique constraint such that one particular column has a value only once?

For instance

-----------------------    
name | pri         


        

Answer 1

I have solved this problem with a strange aproch, using TIMESTAMP as a flag.

That`s usead as a "deleted" FLAG.

If IS NULL, then the register it's not deleted - Instead, if is deleted (<> NULL), the unique key will never conflict avoiding duplicate registers for non flaged as deleted registers.

For example in your case:

You will never have 2 rows with default: NULL, but you can have N with TIMESTAMP with they exclude time.