Editing Relational model (section)

=== Example ===

If a relation of Employees contains the attributes ''{Name, ID}'', then the tuple ''{Alice, 1}'' represents the proposition: "There exists an employee named ''Alice'' with ID ''1''". This proposition may be true or false. If this tuple exists in the relation's body, the proposition is true (there is such an employee). If this tuple is not in the relation's body, the proposition is false (there is no such employee).<ref name="professionals"/>{{rp|96–97}}

Furthermore, if ''{ID}'' is a key, then a relation containing the tuples ''{Alice, 1}'' and ''{Bob, 1}'' would represent the following [[contradiction]]:
# There exists an employee with the name ''Alice'' and the ID ''1''.
# There exists an employee with the name ''Bob'' and the ID ''1''.
# There do not exist multiple employees with the same ID.

Under the [[principle of explosion]], this contradiction would allow the system to prove that any arbitrary proposition is true. The database must enforce the key constraint to prevent this.<ref name="professionals"/>{{rp|104}}