Entity Integrity Constraints
The primary key attribute, PK, of any relational schema R in a database cannot have null values in any tuple. In other words, for each table in a DB, there must be a key; for each key, every row in the table must have non-null values. This is because PK is used to identify the individual tuples.
Mathematically, t[PK] != NULL for any tuple t € r( R).