Experts’ Perspective:Subqueries for Superheroes

Tracy McKibben

DBA Supervisor, Pearson VUE

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Your host

• Richard Douglas

• Sales Engineer / SQL Evangelist

• Maidenhead SQL User Group Leader

• Blog: http://SQL.RichardDouglas.co.uk

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• Email: Richard.Douglas@Software.Dell.com

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Tracy McKibben

DBA Supervisor, Senior SQL Server DBA

Pearson VUE

Blog: realsqlguy.com

Twitter: @RealSQLGuy

I’m not saying I’m Batman, I’m just saying that nobody has

ever seen me and Batman in the same room together...

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Today’s Lineup

• What is a subquery?

• Correlated vs non-correlated

• Occupational hazards

• Subqueries in disguise

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What is a subquery?

A query wrapped within, embedded within, or otherwise referenced by another query. Also known as an INNER query.

The lowly sidekicks of the SQL world.

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Anatomy of a subquery

Which of these is a valid place to put a subquery?

SELECT ?

FROM ?

INNER JOIN ?

ON ?

WHERE ?

GROUP BY ?

HAVING ?

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Anatomy of a subquery

SELECT <a subquery can go here>

FROM <or here>

INNER JOIN <or here>

ON <or here>

WHERE <or here>

GROUP BY <or here>

HAVING <or here>

It’s, like, the ultimate superpower!

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Is this a subquery?

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Is this a subquery?

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Is this a subquery?

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Is this a subquery?

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Is this a subquery?

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Is this a subquery?

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Is this a subquery?

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Is this a subquery?

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Exercise Time!

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Sometimes loopy

A subquery is either correlated or non-correlated.

What is the difference?

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Sometimes loopy

A subquery is either correlated or non-correlated.

What is the difference?

A correlated subquery depends on the

outer query, looping through the outer

resultset, executing once for each row

in the outer query.

A non-correlated subquery stands

alone, only running once, independent

of the values in the outer query.

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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What’s the correlation?

Does this subquery stand alone (non-correlated), or does it need help (correlated)?

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Exercise Time!

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Occupational hazards

Subqueries offer a lot of power and flexibility.

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Occupational hazards

Subqueries offer a lot of power and flexibility.

But

sometimes,

things go

terribly

wrong

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COUNT vs EXISTS

Are there any Green Lantern symbols in this collection?

How do you know?

Did you count them, or did you simply see green and say “yes”?

Did you do this:

WHERE COUNT() > 0?

or, did you do this:

WHERE EXISTS()?

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NOT IN vs NULL

Be careful of NULL values returned by inner query or value list in a NOT IN clause. NULL has no value and can produce surprising results.

Always returns an empty resultset.

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MAX vs RANK

MAX/MIN are often used in a subquery to determine the oldest/greatest/most recent row in the outer query.

RANK/OVER and other windowing functions are more efficient. or

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Exercise Time!

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Subqueries In Disguise

Some T-SQL constructs behave much like subqueries.

• non-indexed views

• common table expressions (CTE)

• user-defined functions

Be wary of performance problems.

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Exercise Time!

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The conclusion

• correlated vs non-correlated - know the difference and potential impacts

• multiple ways to get the same data, but not all perform well

• know new language features like RANK

• don’t count unless you need a number

• be wary of NULL values and hidden subqueries

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Join us at these upcoming SQL events!

SQL Community Corner

Date Location

June 25PASS DBA Virtual Chapter – “The Day After Tomorrow; Why You Need to Baseline”

July 12 SQLSaturday #312 - Sacramento 2014

July 17-19 SQLBits XII – Telford, UK

July 26 SQLSaturday #302 – Albany 2014

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Visit our SQL Community on ToadWorld-www.toadworld.com/platforms/sql-server/default.aspx

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Questions?

Continue the conversation on Twitter via #DellSQL

Thank You!