Keys

Primary Key (PK)

A primary key is a unique column in a particular table. This is the first column in each of our tables. Here, those columns are all called id, but that doesn't necessarily have to be the name. It is common that the primary key is the first column in our tables in most databases.

Foreign Key (FK)

A foreign key is a column in one table that is a primary key in a different table. We can see in the Parch & Posey ERD that the foreign keys are:

1. region_id
2. account_id
3. sales_rep_id

Each of these is linked to the primary key of another table. An example is shown in the image below:

Primary - Foreign Key Link

In the above image you can see that:

1. The region_id is the foreign key.
2. The region_id is linked to id - this is the primary-foreign key link that connects these two tables.
3. The crow's foot shows that the FK can actually appear in many rows in the sales_reps table.
4. While the single line is telling us that the PK shows that id appears only once per row in this table.

If you look through the rest of the database, you will notice this is always the case for a primary-foreign key relationship. In the next concept, you can make sure you have this down!