Формат
<referential constraint definition> ::=
FOREIGN KEY (<referencing columns>)
<references specification>
<references specification> ::=
REFERENCES <referenced table and columns>
<referencing columns> ::=
<reference column list>
<referenced table and columns> ::=
<table name> [(<reference column list>)]
<reference column list> ::=
<column name> [{,<column name>}...]
