Entity Integrity

The primary key attributes $PK$ of each relation schema $R$ in $S$ cannot have null values in any tuple of $r(R)$.

• This is because primary key values are used to identify the tuples
• $t[PK]\ne \text{null}$ for any tuple $t$ in $r(R)$
• if $PK$ has several attributes, $\text{null}$ is not allowed in any of these attributes