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Join question in SQL Server - create statement

Posted on 2013-01-28
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I have a table called tblcategory. It has all of my categories like below:

tblcategory
catnumber  category
 1                    AS400
 2                    Network
 3                    Telecommuincations


my second table is subcategories. This is like so:

tblsubcategory
scatnumber  scatname
1                   User
2                   Printer
3                   Password
4                   Program
5                   Software

my last table is the link between the 2. It is called tblLinkCategory. It contains all of the subcategories per a category. So it is set like this:

tblLinkCategory
lnkcatnumber (which is the category)  lnkscatnumber (which is the subcategory)
   1                                                                1
   1                                                                 4
   2                                                                2
   2                                                                3
   3                                                                 5

What I'd like to do is get a list - with a sql stmt - and read the tbllinkcategory and show me, by name what subcategories are found for a particular category. I struggle with joins so I'm not sure where to start.

I'd also like the sql stmt to show be the subcategories that weren't found connected to a particular category as well. I am using t-sql.

Thanks so much in advance.
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Question by:samic400
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6 Comments
 
LVL 23

Accepted Solution

by:
Ioannis Paraskevopoulos earned 2000 total points
ID: 38827087
Hi,

Try the following for search with category name:

DECLARE @CategoryName VARCHAR(50)
SET @CategoryName = 'AS400' --Or whatever you like

SELECT  C.category,
		SC.scatnam
FROM	tblcategory C
		JOIN tblLinkCategory LC
			ON C.catnumber = LC.lnkcatnumber
		JOIN tblsubcategory SC
			ON SC.scatnumber = LC.lnkscatnumber
WHERE	C.category = @CategoryName

Open in new window


or for search with category number

DECLARE @CategoryNumber INT
SET @CategoryNumber = 1 --Or whatever you like

SELECT  C.category,
		SC.scatnam
FROM	tblcategory C
		JOIN tblLinkCategory LC
			ON C.catnumber = LC.lnkcatnumber
		JOIN tblsubcategory SC
			ON SC.scatnumber = LC.lnkscatnumber
WHERE	C.catnumber = @CategoryNumber

Open in new window


and for search of sub categories with no master categories:

SELECT  SC.scatnumber,
		SC.scatnam
FROM	tblsubcategory SC
		LEFT JOIN tblLinkCategory LC
			ON SC.scatnumber = LC.lnkscatnumber
		LEFT JOIN tblcategory C
			ON C.catnumber = LC.lnkcatnumber
WHERE	C.catnumber IS NULL

Open in new window


Regards,
Giannis
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LVL 23

Expert Comment

by:Ioannis Paraskevopoulos
ID: 38827109
By the way,  

here is a nice visual presentation of joins:

http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html

:)

Giannis
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LVL 6

Expert Comment

by:esolve
ID: 38827125
SELECT
      lnkcatnumber --catnumber
      ,lnkscatnumber  --subcatnumber
      ,cat.Category as CategoryName
      ,sub.scatName as SubCategoryName
FROM
      tblLinkCategory lnk
      INNER JOIN tblcategory cat ON lnk.lnkcatnumber = cat.catnumber  
      INNER JOIN tblsubcategory sub ON lnk.lnkscatnumber = sub.scatnumber
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LVL 69

Expert Comment

by:Scott Pletcher
ID: 38827357
FYI, the typical / "standard" naming for such relationship tables is to use both main table names/entities together, rather than "link".

CategorySubcategory

That inherently implies the link between them.

Or, if you absolutely insist:
tblCategorySubcategory
[Most people long, long ago dropped the "tbl" prefix, in galaxies both near and far, far away.]

Naming the actual link is much more useful than just saying "link".  Also, a single table can link out to several other tables, so "link" is inadequate anyway.
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LVL 13

Author Closing Comment

by:samic400
ID: 38827563
Thanks for the help -
0
 
LVL 32

Expert Comment

by:awking00
ID: 38828236
>>I'd also like the sql stmt to show be the subcategories that weren't found connected to a particular category as well. I am using t-sql. <<
In your example all of the categories and all of the subcategories show up in the tbllinkcategory table, so each subcategory ties to a category. Perhaps you can provide some more data that covers all of the possible criteria you need to meet and your expected output. I can envision output that shows all subcategories with nulls where no category is met, or another output that shows all the categories with an aggregrated string of the subcategories which would be null if none existed,  or still another output with a column for the category and columns for each subcategory and nulls where no matches exist, and there are probably other possibilities as well.
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