SQL | Basic joins exercise | Create multiple joins to show the shortest Doctor Who episodes

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Software ==> SQL  (198 exercises)
Version ==> Any version of SQL Server
Topic ==> Basic joins  (8 exercises)
Level ==> Harder than average
Subject ==> SQL training
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If you haven't already done so, run the script in the above folder to generate the Doctor Who database.

Create a query to list out the appearances of enemies in episodes which have length under 40 characters, where the length is defined as the sum of:

  • the number of characters in the author's name;
  • the number of characters in the episode's title;
  • the number of characters in the doctor's name; and
  • the number of characters in the enemy's name.

This should reveal two episodes:

Two shortest episodes

The two episodes which make it under the very low bar.

You are now a master (or mistress) of joins!  Save this as Time for a coffee, then close it down.

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This page has 2 threads Add post
15 May 22 at 21:43

The proposed solution was to sum the columns at the select part, then do the same when doing the WHERE. I was thinking about other solutions, but after carefully considering my solution: (where I encapsulated the result of the joins, then used the WHERE to filter) I'm starting to think this could create a very bad performance problem because it basically return all rows from the joins.

select * from (
        aut.AuthorName, e.Title, doc.DoctorName, enemy.EnemyName,
            len(aut.AuthorName) +
            len(e.Title) +
            len(doc.DoctorName) +
        ) AS [Total Length]
    tblAuthor aut
    inner join tblEpisode e
        on e.AuthorId = aut.AuthorId
    inner join tblDoctor doc
        on doc.DoctorId = e.DoctorId
    inner join tblEpisodeEnemy epen
        on E.EpisodeId = EPEN.EpisodeId
    INNER JOIN tblEnemy enemy
        on epen.EnemyId = enemy.EnemyId
) AS r
    r.[Total Length] < 40

My question is... would you please say if my solution is bad, and what would be the advantage of calculating during selection then repeating the calculation on the where again... thank you for all your help!

16 May 22 at 09:47

To be honest I don't know!  My gut feeling is that it would be better to do the calculation once only, as you've done.  I'll throw this question open - anyone else out there know the answer?

27 May 22 at 08:23

SELECT  tblAuthor.AuthorName, 

    LEN(tblEnemy.EnemyName))  AS  [Total Length]
FROM  tblEpisodeEnemy

tblEnemy ON tblEpisodeEnemy.EnemyId = tblEnemy.EnemyId


tblEpisode ON tblAuthor.AuthorId = tblEpisode.AuthorId

tblDoctor ON tblEpisode.DoctorId = tblDoctor.DoctorId 
ON tblEpisodeEnemy.EpisodeId = tblEpisode.EpisodeId

WHERE (LEN(tblAuthor.AuthorName)+LEN(tblEpisode.Title)+
    LEN(tblEnemy.EnemyName))  <40





First of all Use Query Editor to Create the joins among the tables tblDoctor, tblAuthor,tblEpisode, tblEnemy,tblEpisodeEnemy 
And then use len function to write query

19 Feb 19 at 21:43

For Amy's choice my enemy name says 'Winifred GillyFlower'.