Inner Join with an example

Please read, Basic SQL Server Interview Questions on Joins article before reading this article.

Inner Join and left join are the most commonly used joins in real time projects. We will talk about left join in a later article. Now, let us understand Inner 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) that have a non null foreign key value(CompanyId in Candidate Table is the foreign key) then we use INNER JOIN. A query involving an INNER JOIN for the Candidate and Company Table is shown below.

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

If we run the above query the output will be as shown in the image below. If you look at the out put, we only got 5 rows. We did not get the 2 rows which has NULL value in the CompanyId column. So an INNER JOIN would get all the rows from the LEFT Table that has non null foreign key value.

Inner Join Result

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

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

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