Unique Keys

[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