Like @ypercube commented, check the
CREATE TABLE page of the manual:
There are three match types:
MATCH PARTIAL, and
(which is the default).
MATCH FULL will not allow one column of
a multicolumn foreign key to be null unless all foreign key columns
are null; if they are all null, the row is not required to have a
match in the referenced table.
MATCH SIMPLE allows any of the foreign
key columns to be null; if any of them are null, the row is not
required to have a match in the referenced table.
MATCH PARTIAL is not
yet implemented. (Of course,
NOT NULL constraints can be applied to
the referencing column(s) to prevent these cases from arising.)
Also, in the chapter on Foreign Keys:
Normally, a referencing row need not satisfy the foreign key
constraint if any of its referencing columns are null. If
is added to the foreign key declaration, a referencing row escapes
satisfying the constraint only if all its referencing columns are null
(so a mix of null and non-null values is guaranteed to fail a
constraint). If you don't want referencing rows to be able to
avoid satisfying the foreign key constraint, declare the referencing
And be sure to consult the current manual. Don't fall for outdated Google links to outdated versions.