What is the difference between a primary key, foreign key, and unique key?

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All three are constraints that enforce uniqueness or referential integrity, but each plays a different role. Primary key — uniquely identifies each row. One per table. NOT NULL and unique. Automatically indexed; usually the table's clustered index in InnoDB. Foreign key — column(s) that reference the primary key of another table. Enforces referential integrity ; can cascade on update/delete. Unique key — ensures all values in the column(s) are distinct. Can have multiple per table. Allows one NULL (or many, depending on engine). Primary = unique + not null + one per table. Unique = unique + allows one null + many per table. Foreign = references another table's primary key. Know CASCADE vs RESTRICT on delete. Composite key for many-to-many junction tables.