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One to Many Relationships
1:38In this lesson, we will build on the information we learned about Keys and Constraints to explain the concept of linking or relating, tables in a database system.
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In the previous stages, we learnt about
the high level concepts such as database
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normalization and set theory.
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We also learned about database keys and
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how they are used to create
database relationships.
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Now, we're going to explore table
relationships more in depth.
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There are three types of
relationships between tables.
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They are called one to one,
one to many, and many to many.
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These types are described by how many
rows can be related to each other
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on either side of the relationship.
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One to many relationships are by far
the most common type of relationship,
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that you'll encounter in database design.
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This type of relationship means that one
row in one table can be related to many
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rows in another table at the other side
of the relationship, but not vice versa.
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The many can only happen in one direction.
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Take this Sales Table for example,
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it has a foreign key to the product
table on the column ProductID.
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When describing this relationship you
could say, a Sale has 1 product but
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a Product can be a many sales.
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If you ever get confused as to
which side is the foreign key,
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an easy trick to remember is that the
foreign key always goes on the many side.
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In this example there are many instances
of the ProductID, 145 on the Sale Table.
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That's the many side,
thus that's the foreign key.
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In our next video, we'll take a look
at the next type of relationship.
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