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.

1. Reasons PRIMARY KEY Cannot Be NULL

1
A primary key must uniquely identify each row — NULL cannot uniquely identify anything.
2
NULL is not a value; it means 'unknown', which breaks uniqueness rules.
3
MySQL automatically enforces PRIMARY KEY columns as NOT NULL.
4
Allowing NULLs would prevent the database from ensuring row uniqueness and referential integrity.

2. What MySQL Does Internally

1
MySQL automatically adds an implicit NOT NULL constraint to PRIMARY KEY columns.
2
If you try to insert NULL into a primary key column, MySQL will throw an error.
3
Composite primary keys also require that none of the columns involved contain NULL.

Example: Attempting to Insert NULL into a Primary Key

In summary, primary keys must be NOT NULL and unique so that each row is reliably identifiable. Allowing NULLs would violate this fundamental rule.

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