Question

let’s say i have 2 tables:

Department(depNum)
Worker(id,deptNum) - key should be id

now i want dept to reference an existing value in Department.
so i write in create table:

CREATE TABLE Worker(
id integer primary key,
dept integer references Department);

my question is, i’ve seen in many examples that you also put foreign key with the references statement. i don’t understand what is primary key for.
does it mean that dept will be also a key on Worker?

thank you

Answer 1

You have two tables:

PLAYER, 
primary key (unique) PK_player_id
player_name
foreignt key to TEAM.team_id FK_team_id

TEAM
primary key (unique) PK_team_id
team_name

Every PLAYER is in exact one TEAM.

The PLAYER has a FOREIGN-KEY to the TEAM (FK_team_id). Also you can delete the TEAM, which will delete all player in it cascading (if configured).

Now you can’t create a player without an existing TEAM, because the database ensures this.

EDIT:
Didn’t you ask for the foreign key?

The primary key is one or more than one column, which will identify on datarow within your database. If jou want to create a foreign key, you have to use a column or more than one column) which is unique.

In my example, there is a unique key (the primary key) for every table, because the name may change. To identify the ‘target’ of the foreign key, it has to be unique. so it is liklye to use the prmary key of the second table. (TEAM.PK_team_id)