Key vs. Index in MySQL Help Posted 14 hours ago 2) Only if you used a FOREIGN KEY clause with the table. Otherwise it's just a concept (see #5). 5) No. A "key" is not a real thing. An index is a real thing, a constraint is a real thing, a primary key and a unique key are real things, a foreign key may or may not be a real thing (see #2), but a mere "key" is just a concept. 6) No. An index can exist on a column without that column being a "key" or having a constraint.