Friday, 17 January 2014

Right Outer Join with an example

Inner Join and left join are the most commonly used joins in real time projects. 

Now, let us understand Right Outer join with an example.

Create 2 tables Company and Candidate. Use the script below to create these tables and populate them. CompanyId column in Candidate Table is a foreign key referencing CompanyId in Company Table.

    CompanyId TinyInt Identity Primary Key,
    CompanyName Nvarchar(50) NULL

INSERT Company VALUES('Microsoft')

    CandidateId tinyint identity primary key,
    FullName nvarchar(50) NULL,
    CompanyId tinyint REFERENCES Company(CompanyId)

INSERT Candidate VALUES('Ron',1)
INSERT Candidate VALUES('Pete',2)
INSERT Candidate VALUES('Steve',3)
INSERT Candidate VALUES('Steve',NULL)
INSERT Candidate VALUES('Ravi',1)
INSERT Candidate VALUES('Raj',3)
INSERT Candidate VALUES('Kiran',NULL)

If you want to select all the rows from the LEFT Table ( In our example Candidate Table) that have non null foreign key values plus all the rows from the RIGHT table ( In our example Company Table) including the rows that are not referenced in the LEFT Table, then we use RIGHT OUTER JOIN. A query involving a RIGHT OUTER JOIN for the Candidate and Company Table is shown below.

SELECT Cand.CandidateId, Cand.FullName, Cand.CompanyId, Comp.CompanyId, Comp.CompanyName
FROM Candidate Cand
ON Cand.CompanyId = Comp.CompanyId

If we run the above query the output will be as shown in below. If you look at the out put, we now got 6 rows. All the rows from the Candidate Table that has non null foreign key value plus all the rows from the Company Table including the row that is not referenced in the Candidate Table.

Right Outer Join Results

Instead of using RIGHT OUTER JOIN keyword we can just use RIGHT JOIN keyword as shown below. RIGHT OUTER JOIN or RIGHT JOIN means the same.

SELECT Cand.CandidateId, Cand.FullName, Cand.CompanyId, Comp.CompanyId, Comp.CompanyName
FROM Candidate Cand
RIGHT JOIN Company Comp
ON Cand.CompanyId = Comp.CompanyId

If you can think of any other sql server interview questions please post them as comments, so they will be useful to other users like you. This will be a great help from your side to improve this site.

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