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Review and Practice
2:06The final video in this stage will recap what we learned about subqueries and then back to the SQL playground for some more practice.
We've just learnt some great tools that
make querying with SQL so powerful.
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In most environments subqueries
probably aren't used every day.
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But when you're faced with the task
of putting together a tricky dataset,
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subqueries come in very handy.
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Let's recap what we've learnt.
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There are a couple of different reasons
why you might need to use a subquery.
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You can use a subquery when
you don't necessarily know
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all the values you want to
include in a WHERE clause.
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Or when you need to create
a temporary data view, that
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you can join to other data
elsewhere in your database.
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When using a subquery as part
of an in statement you can put
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any valid statement in parenthesis and
the query puzzle will limit the results of
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the outer query to the values that
are returned by the subquery.
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The key to this approach is that
the subquery can only return
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one column via the SELECT clause.
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The number of ways that you
construct to derive tables is
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only limited by your imagination.
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They sample syntax here only represents
one possible way to use a derived
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table but the things that apply every
time you create a template table include,
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you have to use parentheses
just like any other subquery.
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You need to alias the derived table so
that the outer query can reference it.
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The subquery is part of
the FROM of the outer query and
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you can join to it just
like any other table.
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It's time for another practice session.
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Open up the accompanying
SQL Playground with this video. We're
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still using the library database
with two locations in the North and
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South. In the coding
areas in the top left.
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I have given you a number of reports
that I'd like you to generate.
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Try using subqueries where you can.
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This concludes our course on
Querying Relational Databases.
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You have been presented
with a lot of information.
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Make sure you practice your new
skills in the SQL Playground and
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feel free to keep coming back to brush
up on your skills and solidify them.
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Once you're done practicing,
it's time for your final assessment.
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Good luck, and I'll see you next time.
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