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Unique Keys

2:06 with Andrew Chalkley and Kevin Nahm

Unique Keys are specifically configured so that no value can be repeated within it.

There are 3 types of Database Keys we will be reviewing in this course:

Unique Keys
Primary Keys
Foreign Keys

For more info on Unique Keys, Wikipedia: Unique Keys is a good introduction.

This article from W3 Schools shows How unique keys are implemented in various SQL database platforms

  • 0:00

    [MUSIC]

  • 0:04

    Remember that relational databases are called

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    relational because the tables are connected by common attributes.

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    The attributes are the columns with common data that link the rows of data together.

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    These special columns are called keys.

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    Database keys aren't only used for relating data together.

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    Some keys guarantee that each row is distinct from one another.

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    Other keys guarantee a certain value doesn't repeat within a particular column.

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    We'll look at the three most common database case keys.

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    Unique keys, primary keys and foreign keys.

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    The first key we're going to take a look at is a unique key.

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    This is very much what it sounds like.

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    It's a column specifically configured so that no value can be repeated within it.

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    Let's see it in action.

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    Let's use a simple table to demonstrate a unique key.

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    This is a person table with an id, name and ssn, or Social Security number.

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    A number of rows are inserted into the table.

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    By accident, someone entered in the same Social Security number for

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    Dorian as Steve's.

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    Obviously, this is not good.

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    This Social Security number column is not a unique key.

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    Let's start over again with a person table,

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    where the Social Security number column is a unique key.

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    This instructs the database to enforce uniqueness

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    among all social security number values.

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    Let's see what happens when we add the same data again.

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    Dorian's row is rejected.

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    And this is exactly what we wanted.

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    There's a few other things you should know about unique keys.

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    If the schema permits, they can be null.

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    There can be multiple unique keys per table.

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    For example, a unique email address column

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    could also be in the same table as a unique Social Security number column.

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    Also, values in unique columns can be changed to new values

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    as long as they don't conflict with any existing values in that column.

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