Exercise: Use a CTE to avoid including the same CASE statement twice

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The answer to the exercise will be included and explained if you attend one of more of the courses listed below!

Category ==> SQL  (156 exercises)
Topic ==> Derived tables and CTEs  (8 exercises)
Level ==> Relatively easy
Courses ==> Introduction to 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.

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