[MUSIC] 0:00 In the previous stages, we learnt about the high level concepts such as database 0:04 normalization and set theory. 0:08 We also learned about database keys and 0:11 how they are used to create database relationships. 0:13 Now, we're going to explore table relationships more in depth. 0:17 There are three types of relationships between tables. 0:21 They are called one to one, one to many, and many to many. 0:24 These types are described by how many rows can be related to each other 0:29 on either side of the relationship. 0:33 One to many relationships are by far the most common type of relationship, 0:36 that you'll encounter in database design. 0:40 This type of relationship means that one row in one table can be related to many 0:43 rows in another table at the other side of the relationship, but not vice versa. 0:48 The many can only happen in one direction. 0:53 Take this Sales Table for example, 0:57 it has a foreign key to the product table on the column ProductID. 0:59 When describing this relationship you could say, a Sale has 1 product but 1:04 a Product can be a many sales. 1:09 If you ever get confused as to which side is the foreign key, 1:12 an easy trick to remember is that the foreign key always goes on the many side. 1:17 In this example there are many instances of the ProductID, 145 on the Sale Table. 1:22 That's the many side, thus that's the foreign key. 1:29 In our next video, we'll take a look at the next type of relationship. 1:33