Unique Constraint with conditions in MYSQL

Question

In postgres we have a constraint defined that essentially allows us to limit the number of entries in a table with a certain value to one. We created this constraint:

Answer 1

Actually it exists. Most contraints depend on the table Engine. I think InnoDB supports this.

To do it, you have to add a UNIQUE index with the unique combination:

ALTER TABLE visitor ADD UNIQUE unique_default_visitor( visitor_uid, list_type );