Why Primary Key Columns Cannot Contain NULL in MySQL
A primary key column cannot contain NULL values because the purpose of a primary key is to uniquely identify every row in a table. A NULL value represents 'unknown' or 'missing' data, which cannot be used for unique identification.
A primary key must uniquely identify each row — NULL cannot uniquely identify anything.
NULL is not a value; it means 'unknown', which breaks uniqueness rules.
MySQL automatically enforces PRIMARY KEY columns as NOT NULL.
Allowing NULLs would prevent the database from ensuring row uniqueness and referential integrity.
MySQL automatically adds an implicit NOT NULL constraint to PRIMARY KEY columns.
If you try to insert NULL into a primary key column, MySQL will throw an error.
Composite primary keys also require that none of the columns involved contain NULL.
In summary, primary keys must be NOT NULL and unique so that each row is reliably identifiable. Allowing NULLs would violate this fundamental rule.