One to One Relationship

gistfile1.txt

If you have two tables suppose Users table and a UsersProfile Table.

In your users table you will have things like user_id PRIMARY_KEY AUTO_INCREMENT/SERIAL, email VARCHAR(244), password varchar(12)
In your user_profile table you will have PRIMARY_KEY users.user_id FOREIGN_KEY users.user_id, first_name VARHCAR(122), last_name VARCHAR(133)

The primary and foreign key needs to be referencing the users table. The reason for this because it forces data integrity, and forcing a one to one relationship 
between the two tables.

One-toOne relationships are typically used to split the data that may not always be needed together or to seperate sensitive information into
a seperate table,

Important features to know about foreign_keys, since foreign keys reference another tables primary_key. 
What happens if that tables primary key got deleted? 
We have something on the child table known as ON UPDATE OR ON DELETE associated with a foreign key as part of the foreign key declaration.


We have several things,

Common options include ON UPDATE:
CASCADE: Update the foreign key values based in the dependent tables to match the new primary key value
SET NULL: set the foreign key value(s) in the dependent table(s) to NULL
SET DEFAULT: set the foreign key value(s) in the dependent table(s) to their default value
RESTRICT: Prevent the update if it would violate referential integrity.

ON DELETE: Specifies the action to take when the referenced primary key is deleted.
CASCADE: Delete the dependent rows in the child table(s) along with the referenced row in the parent table.
SET NULL: Set the foreign key value(s) in the dependent table(s) to their default value.