What is a primary key? Can a table have more than one primary key?
Understanding Primary Keys in MySQL
A primary key is a column or a combination of columns that uniquely identifies each row in a table. It ensures that no two rows have the same value in the primary key column(s) and that the value is never NULL.
1. Characteristics of a Primary Key
Uniquely identifies each row in the table.
Cannot contain NULL values.
Automatically creates a unique index.
Ensures entity integrity in relational databases.
2. Can a Table Have More Than One Primary Key?
A table can only have one primary key constraint.
However, that primary key can consist of multiple columns — this is called a composite primary key.
You cannot define two separate primary keys in one table.
Example: Composite Primary Key
In this example, neither student_id nor course_id alone uniquely identifies a row, but the combination does — forming a composite primary key.