A foreign key is one of the different types of Oracle constraints.
The purpose of a constraint is literally to constrain or restrict the quality of data which you are allowed to insert into a table.

There are 5 kinds of constraints :

• Check
• Not-Null
• Unique
• Primary Key
• Foreign Key

Check Constraints

A check constraint allows you to define a function which determines whether the data is allowable or not.

Example: Let’s add a check constraint to employees.surname (table Employees, column Surname)
which will only allow characters to be entered in upper case:

If you try to add a surname in lower case , you will receive the following error:

Now let’s add it the proper way:

Not-Null Constraints

This simple constraint determines whether your column is allowed to contain null data.
By default, a column can contain null data, unless you add a not-null constraint.

Example : Let’s add a not-null constraint to Employees.Surname

Now try to insert a null value.

Unique Constraints

These constraints enforce uniqueness in your column’s data : it will stop duplicated data from being inserted.

Example:

We know there is already an entry named ‘JONES’ in the table, so let’s see what happens when we try to insert it again:

Primary Key Constraints

A primary key constraint is a combination of a unique and a not-null constraint.
It allows you to use a column in your table to uniquely identify each row by your predetermined key.
Creating a primary key automatically creates a unique index of the same name on the table.
This is how Oracle enforces the uniqueness.

Example: Before we can create a primary key, let’s drop the previous not-null and unique key constraints.

Either inserting a pre-existing surname or attempting to insert a null value into the surname column
will now result in an error.

Foreign Key Constraints

Another name for these is relational integrity constraints.
They enforce logical relationships between parent and child tables.

Let’s use a geographical example.

We have a parent table called Countries and a child table called Cities.
A row in the Cities table must be able to find its parent Country in the Countries table
before you are allowed to insert the City.

Parent table Countries :

Child table Cities:

To implement this is slightly more complicated than the other constraints:
First you create the primary key on the parent table:

Now we create the foreign key constraint on the child table which references the primary key on the parent table:

Time to test: let’s try to add a row in Cities with a non-existent Countryid.

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