References can be defined as circular in that Table A can reference Table B, which can reference Table A. At least one set of FOREIGN KEYS must be defined on nullable columns.
If the FOREIGN KEYS in Table A are on columns defined as nullable, then rows can be inserted into Table A with nulls for the FOREIGN KEY columns. After the appropriate rows exist in Table B, the nulls of the FOREIGN KEY columns in Table A can then be updated to contain values that are not null that match the Table B values.