Final answer:
In the SQL statement 'constraint X foreign key (Y) references Z', Y is the column in table X that serves as the foreign key linking to the primary key of table Z. The relationship is such that a record must exist in Z for X to reference, but not vice versa.
Step-by-step explanation:
In SQL, the statement constraint X foreign key (Y) references Z specifies a relationship where Y is the name of the column in table X that is the foreign key. This column Y is used to link to the primary key in another table, which is table Z in this case. Essentially, for each record in table X, there must be a corresponding record in table Z where the value in column Y matches the value in Z's primary key column.
The relationship between tables is such that if there exists a record in table X with a certain value in the foreign key column Y, then there must be a matching record in table Z. However, the existence of a record in table Z does not necessitate a corresponding record in table X. Therefore, the presence of a foreign key value in Y is sufficient to assert the existence of a corresponding primary key in Z.