SQL | Derived tables and CTEs exercise | Use a CTE to avoid including the same CASE statement 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.

You can learn how to do this exercise if you attend one of more of the courses listed below!

Software ==> SQL  (198 exercises)
Version ==> Any version of SQL Server
Topic ==> Derived tables and CTEs  (13 exercises)
Level ==> Relatively easy
Courses ==> Introduction to SQL  /  Fast-track SQL  /  Advanced SQL
Before you can do this exercise, you'll need to download and unzip this file (if you have any problems doing this, click here for help). Once you've done this:
  1. Go into SQL Server Management Studio;
  2. Open the SQL file you've just unzipped (you can press CTRL + O to do this); then
  3. 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 count the number of events by a column called Era which you'll calculate, without including this calculation twice:

Expression to avoid

Here's what NOT to do - we want to avoid repeating the same calculation twice.

To do this, you can do the calculation in two bites, using a CTE to hold the intermediate stage.  First create a query to show the era for each event:

Events by era

You could peek at the expression at the start of this exercise to get help on how to calculate the era!

 

Now store this as a CTE, and write a query using it which shows the number of events per era:

The final result

What the entire query should show when you run it.

 

Save this query as Epoch-making, and 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.
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