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Join Queries
1:47A join is the instruction to the query engine on how to relate records in one table to records in another.
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[MUSIC]
0:00
Previously, we've talked
about database concepts.
0:04
Such as primary keys, foreign keys,
0:07
table relationships,
normalization and set theory.
0:10
These concepts lead the foundation of
how we write more powerful SQL queries.
0:14
We will revisit and
0:20
expand on these topics as we progress
through the rest of this course
0:21
and get more hands on with the data.
0:25
In previous courses,
you've seen lots of examples of queries.
0:28
However, they've all been
from one table at a time.
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But, how can you combine related data for
more than one table?
0:35
You can use something that
is called a table join.
0:40
Using a join in a query tells
the database to mesh the data from
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two different tables together
into one result set.
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The two most common types of joins
are inner join and outer join.
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We're going to learn about
each of these in detail.
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Here we see two tables.
0:59
Customer and customer detail.
1:01
If we want to display the results from
both the tables in the same view,
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we use the keyword JOIN
1:08
to tell SQL to put the data together,
only selecting what we want.
1:10
Let's use a JOIN to select the FirstName
and LastName from the customer table
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and join it to the phone and date of
birth from the customer detail table.
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The result set generated from the query
doesn't affect the original two tables.
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All the data is there,
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separated.
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Only the result set is shown
based on the relationship.
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Joins can be used in one to one, one to
many, and many to many relationships.
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In the next video,
we'll look at the first type of join.
1:42
Inner join.
1:46