SQL Server: Multiple table joins with a WHERE clause - sql-server

Question

I'm using SQL Server and I'm having a difficult time trying to get the results from a SELECT query that I want. I've tried joining in different orders and using subqueries but nothing quite works the way I want. Take this contrived example of software applications, with different version levels, that might be installed on peoples computers.
I need to perform a JOIN with a WHERE, but for some reason I can't get the results I want.
Maybe I'm looking at my data wrong, I'm not quite sure why I can't get this to work.
Application table
ID Name
1 Word
2 Excel
3 Powerpoint
Software Table (contains version information for different applications)
ID ApplicationID Version
1 1 2003
2 1 2007
3 2 2003
4 2 2007
5 3 2003
6 3 2007
Software_Computer junction table
ID SoftwareID ComputerID
1 1 1
2 4 1
3 2 2
4 5 2
Computer table
ID ComputerName
1 Name1
2 Name2
I want a query that I could run where I select a specific computer to display what software version and application is has, but I also want it to display what application it does not have(the version would be a NULL since it doesn't have that software on it)
SELECT Computer.ComputerName, Application.Name, Software.Version
FROM Computer
JOIN Software_Computer
ON Computer.ID = Software_Computer.ComputerID
JOIN Software
ON Software_Computer.SoftwareID = Software.ID
RIGHT JOIN Application
ON Application.ID = Software.ApplicationID
WHERE Computer.ID = 1
I want the following result set
ComputerName Name Version
Name1 Word 2003
Name1 Excel 2007
Name1 Powerpoint NULL
But I just get
Results
ComputerName Name Version
Name1 Word 2003
Name1 Excel 2007
I thought the RIGHT JOIN would include all the results in the application table, even if they aren't associated with the computer. What am I missing/doing wrong?

Answer 1

When using LEFT JOIN or RIGHT JOIN, it makes a difference whether you put the filter in the WHERE or into the JOIN.
See this answer to a similar question I wrote some time ago:
What is the difference in these two queries as getting two different result set?
In short:
if you put it into the WHERE clause (like you did, the results that aren't associated with that computer are completely filtered out
if you put it into the JOIN instead, the results that aren't associated with that computer appear in the query result, only with NULL values
--> this is what you want

Answer 2

The third row you expect (the one with Powerpoint) is filtered out by the Computer.ID = 1 condition (try running the query with the Computer.ID = 1 or Computer.ID is null it to see what happens).
However, dropping that condition would not make sense, because after all, we want the list for a given Computer.
The only solution I see is performing a UNION between your original query and a new query that retrieves the list of application that are not found on that Computer.
The query might look like this:
DECLARE #ComputerId int
SET #ComputerId = 1
-- your original query
SELECT Computer.ComputerName, Application.Name, Software.Version
FROM Computer
JOIN dbo.Software_Computer
ON Computer.ID = Software_Computer.ComputerID
JOIN dbo.Software
ON Software_Computer.SoftwareID = Software.ID
RIGHT JOIN dbo.Application
ON Application.ID = Software.ApplicationID
WHERE Computer.ID = #ComputerId
UNION
-- query that retrieves the applications not installed on the given computer
SELECT Computer.ComputerName, Application.Name, NULL as Version
FROM Computer, Application
WHERE Application.ID not in
(
SELECT s.ApplicationId
FROM Software_Computer sc
LEFT JOIN Software s on s.ID = sc.SoftwareId
WHERE sc.ComputerId = #ComputerId
)
AND Computer.id = #ComputerId

Answer 3

try this
DECLARE #Application TABLE(Id INT PRIMARY KEY, NAME VARCHAR(20))
INSERT #Application ( Id, NAME )
VALUES ( 1,'Word' ), ( 2,'Excel' ), ( 3,'PowerPoint' )
DECLARE #software TABLE(Id INT PRIMARY KEY, ApplicationId INT, Version INT)
INSERT #software ( Id, ApplicationId, Version )
VALUES ( 1,1, 2003 ), ( 2,1,2007 ), ( 3,2, 2003 ), ( 4,2,2007 ),( 5,3, 2003 ), ( 6,3,2007 )
DECLARE #Computer TABLE(Id INT PRIMARY KEY, NAME VARCHAR(20))
INSERT #Computer ( Id, NAME )
VALUES ( 1,'Name1' ), ( 2,'Name2' )
DECLARE #Software_Computer TABLE(Id INT PRIMARY KEY, SoftwareId int, ComputerId int)
INSERT #Software_Computer ( Id, SoftwareId, ComputerId )
VALUES ( 1,1, 1 ), ( 2,4,1 ), ( 3,2, 2 ), ( 4,5,2 )
SELECT Computer.Name ComputerName, Application.Name ApplicationName, MAX(Software2.Version) Version
FROM #Application Application
JOIN #Software Software
ON Application.ID = Software.ApplicationID
CROSS JOIN #Computer Computer
LEFT JOIN #Software_Computer Software_Computer
ON Software_Computer.ComputerId = Computer.Id AND Software_Computer.SoftwareId = Software.Id
LEFT JOIN #Software Software2
ON Software2.ID = Software_Computer.SoftwareID
WHERE Computer.ID = 1
GROUP BY Computer.Name, Application.Name

Answer 4

You need to do a LEFT JOIN.
SELECT Computer.ComputerName, Application.Name, Software.Version
FROM Computer
JOIN dbo.Software_Computer
ON Computer.ID = Software_Computer.ComputerID
LEFT JOIN dbo.Software
ON Software_Computer.SoftwareID = Software.ID
RIGHT JOIN dbo.Application
ON Application.ID = Software.ApplicationID
WHERE Computer.ID = 1
Here is the explanation:
The result of a left outer join (or simply left join) for table A and
B always contains all records of the "left" table (A), even if the
join-condition does not find any matching record in the "right" table
(B). This means that if the ON clause matches 0 (zero) records in B,
the join will still return a row in the result—but with NULL in each
column from B. This means that a left outer join returns all the
values from the left table, plus matched values from the right table
(or NULL in case of no matching join predicate). If the right table
returns one row and the left table returns more than one matching row
for it, the values in the right table will be repeated for each
distinct row on the left table. From Oracle 9i onwards the LEFT OUTER
JOIN statement can be used as well as (+).

Answer 5

SELECT p.Name, v.Name
FROM Production.Product p
JOIN Purchasing.ProductVendor pv
ON p.ProductID = pv.ProductID
JOIN Purchasing.Vendor v
ON pv.BusinessEntityID = v.BusinessEntityID
WHERE ProductSubcategoryID = 15
ORDER BY v.Name;

Answer 6

You almost have it. You need a Right join to the application, So it knows that the right table which is application is important
SELECT Computer.ComputerName, Application.Name, Software.Version
FROM Computer
JOIN Software_Computer
ON Computer.ID = Software_Computer.ComputerID
Right JOIN Software
ON Software_Computer.SoftwareID = Software.ID
RIGHT JOIN Application
ON Application.ID = Software.ApplicationID
WHERE Computer.ID = 1

Answer 7

Try this working fine....
SELECT computer.NAME, application.NAME,software.Version FROM computer LEFT JOIN software_computer ON(computer.ID = software_computer.ComputerID)
LEFT JOIN software ON(software_computer.SoftwareID = Software.ID) LEFT JOIN application ON(application.ID = software.ApplicationID)
where computer.id = 1 group by application.NAME UNION SELECT computer.NAME, application.NAME,
NULL as Version FROM computer, application WHERE application.ID not in ( SELECT s.applicationId FROM software_computer sc LEFT JOIN software s
on s.ID = sc.SoftwareId WHERE sc.ComputerId = 1 )
AND computer.id = 1

Answer 8

select C.ComputerName, S.Version, A.Name
from Computer C inner join Software_Computer SC
on C.Id = SC.ComputerId
Inner join Software S
on SC.SoftwareID = S.Id
Inner join Application A
on S.ApplicationId = A.Id ;

Answer 9

SELECT Computer.Computer_Name, Application1.Name, Max(Soft.[Version]) as Version1
FROM Application1
inner JOIN Software
ON Application1.ID = Software.Application_Id
cross join Computer
Left JOIN Software_Computer
ON Software_Computer.Computer_Id = Computer.ID and Software_Computer.Software_Id = Software.Id
Left JOIN Software as Soft
ON Soft.Id = Software_Computer.Software_Id
WHERE Computer.ID = 1
GROUP BY Computer.Computer_Name, Application1.Name