In the following example, we have a Comments table with a composite primary key (PK)
CREATE TABLE Comments ( video_id INT NOT NULL, user_id INT NOT NULL, comment_dt DATETIME NOT NULL, PRIMARY KEY (video_id, user_id, comment_dt), FOREIGN KEY (video_id) REFERENCES Videos(video_id), FOREIGN KEY (user_id) REFERENCES Users(user_id));There appears to be an identifying relationship, as the primary key of Comments includes its foreign keys.
However, watch what happens when you give this Comments table a surrogate primary key of its own:
CREATE TABLE Comments (comment_id SERIAL PRIMARY KEY,video_id INT NOT NULL,user_id INT NOT NULL,comment_dt DATETIME NOT NULL,FOREIGN KEY (video_id) REFERENCES Videos(video_id),FOREIGN KEY (user_id) REFERENCES Users(user_id));Does that get rid of the identifying relationship? Logically, it still seems to be there, so does this really come down to the physical model and whether or not you want to include a composite key? Are identifying relationships even necessary?
