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SQL | Derived tables and CTEs exercise | Create a CTE to make it easier to group by an expression
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.
You can learn how to do this exercise if you attend one of more of the courses listed below!
- 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).
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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 (
-- 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.