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

I never use MySQL but maybe you can create an index like this:

CREATE UNIQUE INDEX list$default$uk ON list_group ((CASE WHEN list_type='default' THEN NULL ELSE visitor_uid END));

Explanation: A unique index should not care about NULL values. Therefore, make sure the index expression returns NULL for every row where list_type <> 'default'.