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SQL | More exotic joins exercise | Create self-joins between 3 levels of family hierarchies
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You can learn how to do this exercise if you attend the course 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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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:
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.
Here's my solution:
(ISNULL(lv1.FamilyName + ' -> ', '') + ISNULL(lv2.FamilyName + ' -> ' , '') + lv3.FamilyName)
AS 'Family Path'
LEFT JOIN tblFamily lv2
ON lv3.ParentFamilyId = lv2.FamilyID
LEFT JOIN tblFamily lv1
ON lv2.ParentFamilyId = lv1.FamilyID
lv1.familyid, lv2.familyid, lv3.familyid
But yeah, I also used LEFT JOIN instead of FULL OUTER JOIN
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].
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.
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
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],
WHEN TopFamily.FamilyName IS NULL
ELSE TopFamily.FamilyName + ' > '
END + ParentFamily.FamilyName + ' > ' + Family.FamilyName
, 'All categories'
) as 'Family path'
tblFamily as family
tblFamily as ParentFamily
ON ParentFamily.FamilyID = Family.ParentFamilyID
tblFamily as TopFamily
ON TopFamily.FamilyId = ParentFamily.ParentFamilyID
I'm having trouble getting this exercise done.
Is there anywhere i can get the solution?
Hello there, this one is kind of funky but here is my solution.
ISNULL('All categories' + ' > ' +
IIF(ParentFamily.FamilyId = 25,'',ParentFamily.[FamilyName]) + ' > ' +
TopFamily.[FamilyName], 'All categories') AS 'Family path'
[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
I solved this way following the instructions of Wise Owl (outer joins).
(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
I came up with this solution:
isnull(case when b.FamilyName = 'All categories' then '' else 'All categories < 'end +b.FamilyName +' < '+ a.FamilyName, 'All categories') as 'Family path'
left join tblFamily b on a.ParentFamilyId = b.FamilyID
Order by a.FamilyName