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Software ==>	SQL (129 exercises)
Topic ==>	Derived tables and CTEs (13 exercises)
Level ==>	Relatively easy
Subject ==>	SQL training

Use a CTE to group a query by a CASE expression without including it twice

This exercise is provided to allow potential course delegates to choose the correct Wise Owl Microsoft training course, and may not be reproduced in whole or in part in any format without the prior written consent of Wise Owl.

Before you can do this exercise, you'll need to download and unzip this file. Once you've done this:

Go into SQL Server Management Studio;
Open the SQL file you've just unzipped (you can press CTRL + O to do this); then
Execute this script.

This will generate the database that you'll need to use in order to do this exercise (note that the database and script are only to be used for exercises published on this website, and may not be reused or distributed in any form without the prior written permission of Wise Owl).

The aim of this exercise is to show the number of events whose descriptions contain the words this and/or that:

Only 3 events have the holy grail: both THIS and THAT.

To do this you can find all events whose EventDetails column contains the word this or that respectively, using the LIKE keyword.

In a single query solution you would have to use a CASE expression to determine the value of IfThis and IfThat, then group by the same CASE expression. This is messy, and makes it hard to make subsequent changes to your expression (as you have to do it in two places). Read on!

Create a query to solve this problem in two passes:

Pass	What to do
1	Create a CTE (called ThisAndThat?) to determine the values of the IfThis and IfThat flags for each event.
2	Use this CTE to get the required results, as shown at the start of this exercise.

As an aide-memoire, here is the syntax for a CTE:

-- create CTE

WITH CteName AS (

SELECT ...

)

-- then immediately use it

SELECT ... FROM CteName

If you get this working and still have spare time, try changing or extending your query to show the 3 events whose details contain both this and that:

Other than the text they contain, there's no obvious link between these 3 events.

Optionally, save this query as This and that.sql, then close it down.

You can unzip this file to see the answers to this exercise, although please remember this is for your personal use only.

You can find other training resources for the subject of this exercise here:

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EXERCISE TOPIC

SQL EXERCISES

SQL EXERCISES

Use a CTE to group a query by a CASE expression without including it twice

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