Question

I’m trying to create my first database relation and I’m confused here (I am doing lot’s of reading too but I am still confused)
I’m trying to create two tables, venues and venue_types.

‘venues’ has an ID and a VENUE_TYPE column while ‘venue_types’ has ID and TYPE_NAME.
I want the VENUE_TYPE column in ‘venues’ to reference the ID column in ‘venue_types’. Let’s say that the VENUE_TYPE is set to 3, and row three in ‘venue_types’ is Casino.

So in a way this is a one to many relationship table where ‘venue_types’ can have many venues while ‘venues’ can have only one ‘venue_type’.

This is what I have and I’m not sure if it should be the other way around, please let me know:

Thanks a lot everyone :)!

Edit: I am also confused that "venue_type" column has a Foreign key label beside it which I’m not sure if that means it contains a foreign key or that it is a foreign key itself?

Answer 1

In general, the table containing the referencing column (in your example, Venues) is the ‘child’ table; the table containing the referenced column (in your example, Venue_Types) is the ‘parent’ table. Normally, the referenced column is the primary key of the table; it should be a unique identifier or candidate key for the table.

The venue_type column in the Venues table is marked FK because it is a foreign key column; it contains values which must be present in the referenced table, the Venue_types table. You can have compound foreign keys if the referenced table has a compound primary key.

Note that you can have a table with multiple candidate keys, but a table can only have one primary key. Consider the ‘Table of Elements’; it might be represented by:

CREATE TABLE elements
(
    atomic_number   INTEGER NOT NULL PRIMARY KEY CONSTRAINT c1_elements
                    CHECK (atomic_number > 0 AND atomic_number < 120),
    symbol          CHAR(3) NOT NULL UNIQUE CONSTRAINT c2_elements,
    name            CHAR(20) NOT NULL UNIQUE CONSTRAINT c3_elements,
    atomic_weight   DECIMAL(8,4) NOT NULL,
    period          SMALLINT NOT NULL
                    CHECK (period BETWEEN 1 AND 7),
    group           CHAR(2) NOT NULL
                    -- 'L' for Lanthanoids, 'A' for Actinoids
                    CHECK (group IN ('1', '2', 'L', 'A', '3', '4', '5', '6',
                                     '7', '8', '9', '10', '11', '12', '13',
                                     '14', '15', '16', '17', '18')),
    stable          CHAR(1) DEFAULT 'Y' NOT NULL
                    CHECK (stable IN ('Y', 'N'))
);

Each of the columns Symbol, Name and Atomic_Number is a perfectly good candidate key. I chose to use Atomic_Number as the primary key (most useful for isotopes and nuclear physics), but if I was more concerned with chemistry, then Symbol would be a better choice.