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Querying Relational Databases

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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 them.

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

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    [MUSIC] 0:00
    Remember that relational databases are called 0:04
    relational because the tables are connected by common attributes. 0:07
    The attributes are the columns with common data that link the rows of data together. 0:11
    These special columns are called keys. 0:17
    Database keys aren't only used for relating data together. 0:19
    Some keys guarantee that each row is distinct from one another. 0:23
    Other keys guarantee a certain value doesn't repeat within a particular column. 0:27
    We'll look at the three most common database case keys. 0:33
    Unique keys, primary keys and foreign keys. 0:36
    The first key we're going to take a look at is a unique key. 0:40
    This is very much what it sounds like. 0:43
    It's a column specifically configured so that no value can be repeated within it. 0:45
    Let's see it in action. 0:51
    Let's use a simple table to demonstrate a unique key. 0:53
    This is a person table with an id, name and ssn, or Social Security number. 0:56
    A number of rows are inserted into the table. 1:02
    By accident, someone entered in the same Social Security number for 1:05
    Dorian as Steve's. 1:09
    Obviously, this is not good. 1:11
    This Social Security number column is not a unique key. 1:13
    Let's start over again with a person table, 1:18
    where the Social Security number column is a unique key. 1:20
    This instructs the database to enforce uniqueness 1:24
    among all social security number values. 1:27
    Let's see what happens when we add the same data again. 1:30
    Dorian's row is rejected. 1:34
    And this is exactly what we wanted. 1:36
    There's a few other things you should know about unique keys. 1:39
    If the schema permits, they can be null. 1:43
    There can be multiple unique keys per table. 1:46
    For example, a unique email address column 1:49
    could also be in the same table as a unique Social Security number column. 1:53
    Also, values in unique columns can be changed to new values 1:58
    as long as they don't conflict with any existing values in that column. 2:02

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