Difference Between PRIMARY KEY and UNIQUE Key in MySQL
Both PRIMARY KEY and UNIQUE constraints ensure that values in a column (or set of columns) are unique. However, they differ in rules, behavior, and intended use.
A table can have only one PRIMARY KEY, but it can have multiple UNIQUE keys.
PRIMARY KEY columns cannot contain NULL, while UNIQUE key columns can contain one NULL value (per column).
PRIMARY KEY is used to uniquely identify rows; UNIQUE keys enforce uniqueness but are not required for identification.
PRIMARY KEY automatically creates a clustered index (in InnoDB), while UNIQUE creates a secondary index.
Use PRIMARY KEY for the main identifier of the row.
Use UNIQUE when a column must be unique but is not the main identifier (e.g., email, username, phone number).
In summary, every PRIMARY KEY is unique, but not every UNIQUE key is a primary key. PRIMARY KEY identifies the row; UNIQUE simply prevents duplicates.