Link With A Part Of A Table

Tell me please how can I make a foreing key constraint for a table using from another not all primary key or not all data. For example, I need to link a table "subject" with primary key "dscp_num, depart" using depart as a foreing key. But there is no table with such primary key, there is only table "codif_values" where selecting records by condition "codif_num = 1" gives the relation with necesary primary key to link. Also I need to link "subject" table as a primary key table with the table "ses_curr". But this table doesn't include "depart" field. It includes field "reg_num" that is a primary key in table "students". This table include field "stgroup" as a foreing key for table "groups". The last table includes "speciality" field that is a foreing key for table "specialities". And only this table includes "depart" field. Tell me please how I can make a foreing key constraint for table "ses_curr" by table "subject".