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Software ==>	SQL (203 exercises)
Topic ==>	More exotic joins (5 exercises)
Level ==>	Harder than average
Subject ==>	SQL training

Create 2 self-joins between a table of families and itself, to show families, their parents and their grandparents

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).

First open the SQL file in the above folder in Management Studio, and execute it. This should generate a table called tblFamily.

Don't forget that you may need to refresh your tables to see this.

Each row in this table contains a column called ParentFamilyId, which tells you which parent family any family belongs to (the All categories family - number 25 - has no parent, and so sits at the top of the hierarchy).

Create a query which links 3 tables using outer joins as follows:

Table	Alias
tblFamily	Family
tblFamily	ParentFamily
tblFamily	TopFamily

Add calculated columns to your query so that it displays all 25 familes:

If you display the families in FamilyName order, you should see something like this.

You'll need to trap for nulls, for those occasions when a family doesn't have a parent.

Save this query as Happy families, 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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From:	derloopkat
When:	10 Jun 24 at 14:40

SELECT child.FamilyName, IIF(grand_parent.FamilyID is not null, concat(grand_parent.FamilyName, ' > ', parent.FamilyName, ' > ', child.FamilyName)

, IIF(parent.FamilyID is not null, concat(parent.FamilyName, ' > ', child.FamilyName), '')) as FamilyPath

FROM tblFamily child

LEFT JOIN tblFamily parent ON child.ParentFamilyId=parent.FamilyID

LEFT JOIN tblFamily grand_parent ON parent.ParentFamilyId=grand_parent.FamilyID

ORDER BY FamilyName

From:	Orin
When:	02 Mar 24 at 16:18

This one hurt! 5-6 tries and around 3-4 hours to break it. And I doubt I would have succeed without reading the answer 10 times.

To those who make it with only one JOIN + ISNULL : it is pretty, I did not think of it, but to me it is not the point of the exercise as this solution would be harder to scale up it there were more hierarchy levels.

---

To Wise Owl, if you have some 2 min for feedback ...

Short version: for the whole Join topic, I would recommend more, short but tricky, exercises, on very small tables to start with.

The "I have too much free time" version:
I made the mistake of not being 100% focused when I learned the Joins and/or not doing enough exercises right away.
I watched the video several times but somehow, it did not "print" correctly from the beginning. I learned too mechanically, without thinking it through, as it seemed obvious to link 2 identical fields in 2 different tables.
Coming from Excel, at first I saw it as a VLOOKUP giving back all the corresponding lines. No biggy.

I came back at it several times, redid the exercises with a clear mind, hit a wall when you threw a triple tables exercise in the View section. I leveled up when I found, and really studied.learned, pictures with the 7 Venn possible diagrams. And when I accepted they cannot be all done without using WHERE for some. Coming from Excel/VBA, it was not obvious to have to combine 2 instructions.

When I really tried this last one, I thought I was fully prepared. I had done my homework, I had run, I had sweat. I could hear Eyes of the tiger in my mind... and I failed miserably :-)) Why ?
a) I am not that smart and, again, was not focused enough + I tried to do it too fast although it is clearly a conceptual one.
b) It is less the Self part that threw me out rather than joining two fields that were not identical. I still have to digest this join "parent to kid" although the other way does lead to duplicate as each parent list its kid.
c) I do think I missed something by not learning & playing with smaller tables at the very beginning, and doing more small exercises.

For me, this exercise would be as conceptually hard with only 7-8 lines and/or with names embedding clear/familiar clues of the hierarchy (Granpa, Mom, Dad, Kid 1... or Big boss C level / Little bosses B level / Workers A level). It might be easier for some of us to get and read it.

And then, you can throw a second exercise with more lines or 4 hierarchy levels, or other variations... if there are any.

Right now I can't quite see something else than this "Bottom Up" Kids/Parents code approach.

A basic one would be a mirror version with a RIGHT OUTER. As I named my table 123, it ends looking more logical/making slightly more sense given the end-goal... although deep down, it is the same gymnastic mindset that I am not sure I would quickly replicate. I need to move on and create my own tables to redo it 30 times:

USE WorldEvents

SELECT

f1.FamilyName

,f2.FamilyName

,f3.FamilyName

,ISNULL(

ISNULL(f1.FamilyName + ' > ' + f2.FamilyName + ' > ' + f3.FamilyName,

f2.FamilyName + ' > ' + f3.FamilyName),

f3.FamilyName)

FROM tblFamily as f1

RIGHT OUTER JOIN tblFamily as f2

ON f1.FamilyID = f2.ParentFamilyId

RIGHT OUTER JOIN tblFamily as f3

ON f2.FamilyID = f3.ParentFamilyId

From:	Rupchand
When:	11 Jan 24 at 10:20

select family.FamilyName,

isnull(

case when family.FamilyName ='All Categories'

then ''

else 'All categories > '

end + parentfamily.FamilyName +' > ' + family.FamilyName,'All Categories')

as [family path]

from tblfamily family

left join

tblfamily parentfamily

on family.ParentFamilyId = parentfamily.FamilyID

order by FamilyName

From:	FloridaMan
When:	26 Sep 21 at 13:43

Not sure if I can post this, BUT I found a great video explaining this.

https://youtu.be/ck8mVDOOCCg

From:	ShukiMolk
When:	11 Apr 20 at 22:07

Here's my solution:

SELECT
   lv3.FamilyName,
   (ISNULL(lv1.FamilyName + ' -> ', '') + ISNULL(lv2.FamilyName + ' -> ' , '') + lv3.FamilyName)
   AS 'Family Path'
FROM
   tblFamily lv3
   LEFT JOIN tblFamily lv2
   ON lv3.ParentFamilyId = lv2.FamilyID
       LEFT JOIN tblFamily lv1
       ON lv2.ParentFamilyId = lv1.FamilyID
ORDER BY
lv1.familyid, lv2.familyid, lv3.familyid

But yeah, I also used LEFT JOIN instead of FULL OUTER JOIN

From:	hjay629
When:	20 Feb 19 at 06:12

This question is tricky because there are many ways you can do it. In fact, you don't even have to use "null traps", nor all the aliases and joins mentioned in the problem. However, I am not 100% sure this is the way Wise Owl wanted it to be done nor am I sure this is correct. Proceed with caution!

ISNULL(Expression, 'All categories') Function

Checks if any of the arguments in the function is NULL, in this case whether if there is a family of a higher hierarchy, and returns the Top Hierarchy 'All categories'.

Note: The program reads the hierarchy from Lowest->Highest. If there are only two levels the TopFamily becomes NULL and if only one level the TopFamily and ParentFamily become NULL , so on. Family < Parent Family < Top Family [Lowest< Middle < Highest].

CASE

I used a CASE here to check if ONLY the TopFamily is NULL, if it's NULL it will return '' and if it's not NULL it will return the rest of the function if it exists. I did this so that the ISNULL() function does not make every row with less than 3 levels of hierarchy = 'All categories'.

The rest of the function is simple, a basic string calculation to add the remainder of the FamilyNames in the hierarchy.

LEFT JOIN

TopFamily being the highest in the hierarchy, I set it as ParentFamily's ParentFamilyID and Family's ParentFamilyID as ParentFamily's FamilyID. Thus, TopFamily.FamilyID = ParentFamily.ParentFamilyID and Family.ParentFamilyID = ParentFamily.FamilyID.

You can see this more clearly by executing the code below, where I wrote it so that it generates all the revelant FamilyIDs and ParentFamilyIDs

---SPOILER [ANSWER]---

SELECT
---FOR CLARITY---
TopFamily.FamilyID as [TopFamily FamilyID],
ParentFamily.ParentFamilyID as [ParentFamily ParentFamilyID],
ParentFamily.FamilyID as [ParentFamily FamilyID],
Family.ParentFamilyId as [Family ParentFamilyID],
Family.FamilyID as [Family FamilyID],
Family.FamilyName,

ISNULL(
       CASE
WHEN TopFamily.FamilyName IS NULL
THEN ''
ELSE TopFamily.FamilyName + ' > '
       END + ParentFamily.FamilyName + ' > ' + Family.FamilyName

       , 'All categories'


) as 'Family path'
FROM
tblFamily as family
LEFT JOIN
tblFamily as ParentFamily
ON ParentFamily.FamilyID = Family.ParentFamilyID
LEFT JOIN
tblFamily as TopFamily
ON TopFamily.FamilyId = ParentFamily.ParentFamilyID
ORDER BY

FamilyName

From:	odkao
When:	29 Jun 18 at 14:17

Hello.

I'm having trouble getting this exercise done.

Is there anywhere i can get the solution?

From:	Phithel
When:	19 Oct 18 at 21:10

Hello there, this one is kind of funky but here is my solution.

USE WorldEvents

SELECT
   Family.[FamilyName],
   ISNULL('All categories' + ' > ' +
   IIF(ParentFamily.FamilyId = 25,'',ParentFamily.[FamilyName]) + ' > ' +
   TopFamily.[FamilyName], 'All categories') AS 'Family path'
FROM
   [dbo].[tblFamily] AS Family
   LEFT JOIN [dbo].[tblFamily] AS ParentFamily
       ON Family.ParentFamilyId = ParentFamily.FamilyID
   LEFT JOIN [dbo].[tblFamily] AS TopFamily
       ON Family.FamilyId = TopFamily.FamilyID
ORDER BY
   Family.[FamilyName]

From:	wagnnerfh
When:	01 Feb 20 at 14:25

Hi.

I solved this way following the instructions of Wise Owl (outer joins).

SELECT Family.FamilyName,
(TopFamily.FamilyName + ' > ' + ParentFamily.FamilyName + ' > ' + Family.FamilyName) AS FamilyPath

FROM tblFamily AS TopFamily

FULL OUTER JOIN tblFamily AS ParentFamily

ON TopFamily.FamilyID = ParentFamily.ParentFamilyId

FULL OUTER JOIN tblFamily AS Family

ON ParentFamily.FamilyID = Family.ParentFamilyId

WHERE Family.ParentFamilyId IS NOT NULL
   AND Family.FamilyName IS NOT NULL
   AND ParentFamily.ParentFamilyId IS NOT NULL
   AND ParentFamily.FamilyName IS NOT NULL
   AND TopFamily.FamilyName IS NOT NULL
   AND TopFamily.FamilyID IS NOT NULL

From:	SmartSqler
When:	13 Jul 18 at 13:30

Hey,

I came up with this solution:

select
a.FamilyName,
isnull(case when b.FamilyName = 'All categories' then '' else 'All categories < 'end +b.FamilyName +' < '+ a.FamilyName, 'All categories') as 'Family path'

from
tblFamily a
left join tblFamily b on a.ParentFamilyId = b.FamilyID

Order by a.FamilyName

From:	SeahawkZim
When:	21 Mar 20 at 19:23

Perfectly elegant. Did you have to spend much time on the answer to this? Or, was it intuitive?

From:	Andy B
When:	29 Jun 18 at 18:43

We don't publish the exercises, I'm afraid - but you never know, some kind person might help ....

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EXERCISE TOPIC

SQL EXERCISES

SQL EXERCISES

Create 2 self-joins between a table of families and itself, to show families, their parents and their grandparents

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