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

You could make a trigger that checks if there allready is a field with the 'TRUE' value, and if so take action.

Note that you cannot easily "reject" the update. (see e.g. : How to abort INSERT operation in MySql trigger? ).

You could for instance just insert it with false, and save your error somehow, by setting a flag.