Friday, 17 January 2014

Full Outer Join in SQL Server with an example


Inner Join and left join are the most commonly used joins in real time projects. It is very important that you understand the basics of joins before reading this article. Please read the articles below if you have not done so already.
1. Basics of Joins in SQL Server
2. Inner Join
3. Left Join
4. Right Join
Now, let us understand Full 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.

CREATE TABLE Company
(
    CompanyId TinyInt Identity Primary Key,
    CompanyName Nvarchar(50) NULL
)
GO

INSERT Company VALUES('DELL')
INSERT Company VALUES('HP')
INSERT Company VALUES('IBM')
INSERT Company VALUES('Microsoft')
GO

CREATE TABLE Candidate
(
    CandidateId tinyint identity primary key,
    FullName nvarchar(50) NULL,
    CompanyId tinyint REFERENCES Company(CompanyId)
)
GO

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)
GO



If you want to select all the rows from the LEFT Table ( In our example Candidate Table ) plus all the rows from the RIGHT table ( In our exampleCompany Table ) , then we use FULL OUTER JOIN. A query involving a FULL 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
FULL OUTER JOIN Company Comp
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 8 rows. All the rows from the Candidate Table and all the rows from the Company Table.


Full Outer Join Result

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

SELECT Cand.CandidateId, Cand.FullName, Cand.CompanyId, Comp.CompanyId, Comp.CompanyName
FROM Candidate Cand
FULL 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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