1
00:00:00,000 --> 00:00:04,268
[MUSIC]

2
00:00:04,268 --> 00:00:08,929
In the previous stages, we learnt about
the high level concepts such as database

3
00:00:08,929 --> 00:00:11,500
normalization and set theory.

4
00:00:11,500 --> 00:00:13,890
We also learned about database keys and

5
00:00:13,890 --> 00:00:17,100
how they are used to create
database relationships.

6
00:00:17,100 --> 00:00:20,010
Now, we're going to explore table
relationships more in depth.

7
00:00:21,120 --> 00:00:24,590
There are three types of
relationships between tables.

8
00:00:24,590 --> 00:00:28,390
They are called one to one,
one to many, and many to many.

9
00:00:29,660 --> 00:00:33,830
These types are described by how many
rows can be related to each other

10
00:00:33,830 --> 00:00:36,490
on either side of the relationship.

11
00:00:36,490 --> 00:00:40,650
One to many relationships are by far
the most common type of relationship,

12
00:00:40,650 --> 00:00:43,480
that you'll encounter in database design.

13
00:00:43,480 --> 00:00:48,160
This type of relationship means that one
row in one table can be related to many

14
00:00:48,160 --> 00:00:53,340
rows in another table at the other side
of the relationship, but not vice versa.

15
00:00:53,340 --> 00:00:56,010
The many can only happen in one direction.

16
00:00:57,130 --> 00:00:58,900
Take this Sales Table for example,

17
00:00:59,940 --> 00:01:04,570
it has a foreign key to the product
table on the column ProductID.

18
00:01:04,570 --> 00:01:09,880
When describing this relationship you
could say, a Sale has 1 product but

19
00:01:09,880 --> 00:01:12,490
a Product can be a many sales.

20
00:01:12,490 --> 00:01:17,060
If you ever get confused as to
which side is the foreign key,

21
00:01:17,060 --> 00:01:22,360
an easy trick to remember is that the
foreign key always goes on the many side.

22
00:01:22,360 --> 00:01:29,890
In this example there are many instances
of the ProductID, 145 on the Sale Table.

23
00:01:29,890 --> 00:01:33,680
That's the many side,
thus that's the foreign key.

24
00:01:33,680 --> 00:01:37,930
In our next video, we'll take a look
at the next type of relationship.