Question

Is it allowed in SQL to have a foreign key that is not a primary key but a (multiple) unique key? To be specific:

[Table 1]

CREATE TABLE Session_Record(
c_id CHAR (3) NOT NULL
    REFERENCES Club_Record(c_id),
sess_id NUMBER (4) NOT NULL CHECK (sess_id >0)
room CHAR (4) NOT NULL,
UNIQUE(c_id, sess_id)
);

[Table 2]

CREATE TABLE SessionDuration__Record(
c_id CHAR (3) NOT NULL
    REFERENCES Club_Record(c_id),
sess_id NUMBER (4) NOT NULL CHECK (sess_id >0)
    REFERENCES Session_Record(sess_id),
day CHAR(9) NOT NULL,
duration NUMBER(3) NOT NULL,
UNIQUE(c_id, sess_id)
);

“c_id” is a primary key in the table Club_Record. But sess_id is not a primary key. It is a multiple candidate key in combination with c_id. Is this form of declaration allowed in SQL?. Because Oracle 10g is returning: “no matching unique or primary key for this column-list” for the (sess_id) foreign key statement.
Help is much appreciated!

Answer 1

Your SQL is invalid in a few different ways (missing a comma after the definition of Session_Record.sess_id; using day as an identifier without quoting it), but sticking just to the problem that you explicitly asked about — I’m guessing what you really want is this:

CREATE TABLE SessionDuration__Record
(
    c_id      CHAR(3)    NOT NULL  REFERENCES Club_Record(c_id),
    sess_id   NUMBER(4)  NOT NULL  CHECK (sess_id >0),
    "DAY"     CHAR(9)    NOT NULL,
    duration  NUMBER(3)  NOT NULL,
    UNIQUE(c_id, sess_id),
    FOREIGN KEY (c_id, sess_id) REFERENCES Session_Record(c_id, sess_id)
);

That is, you want to be sure that each (SessionDuration__Record.c_id, SessionDuration__Record.sess_id) corresponds to an existent (Session_Record.c_id, Session_Record.sess_id).