Almost unique constraint in MySQL Is is possible to have a key in MySQL 5.7, which is unique for all values except one. Let's say -1 is a not assigned wildcard. Is it possible to have a table with the following structure

user | car | seat
1    | 1   | -1
2    | 1   | -1
3    | 1   |  2


That will allow 

INSERT INTO table VALUES (4, 1,  3)
INSERT INTO table VALUES (5, 1, -1)


and fail

INSERT INTO table VALUES (4, 1, 2)


Same applies for updates.
Yes, you can (ab)use NULL for this. Unique indexes allow multiple rows where the unique field is NULL. You just have to make sure you make the field nullable.

Note that this only applies to unique keys, not primary keys - primary keys cannot be nullable.