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In this video, we'll be looking at one more type of table relationship.

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The one to one relationship.

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A one to one relationship
between two tables

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means that one row from one
table can only relate to one row

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on the other table at the other side
of their relationship and vice versa.

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In practice, you don't see this
type of relationship very often.

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But there are a few
occasions when you might.

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Here you can see the one to one
relationship between table A and table B.

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Notice that the ID's one, two, and
three appear only once in each table.

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While there is nothing technically
wrong with this design,

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in practice, it is usually better to
just combine these two tables into one.

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This is because all non ID columns
depend solely on the ID column

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A once one design might come in handy
though separating the frequently and

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infrequently used columns can boost
performance by reducing the overhead

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to access the most
frequently used columns.

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Say you have a database that was
purchased from a third party vendor, and

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you're not allowed to modify their tables,
but

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you want to extend the functionality
of your application by adding columns.

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You would create a new table to extend
the functionality of the core table.

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Let's take a look at an example
that uses a one-to-one

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relationship to extend a table.

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There we have a customer table that
sits in a third party database,

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that our company is purchased when
not allowed to modify the structure

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of the core table but we want to add a few columns.

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We could create a second table and

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call it something that identifies
as a sibling to the customer table.

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Like Customer_ Extension,

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we give it the same primary key
as the original customer table.

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This field ID is also the foreign key
back to the customer table then we can

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add new columns we want in this case
phone and date of birth this relationship

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between the core customer table and
the new extension table is one to one.