Difference between Index Scan and Index Seek and Index Scan:

Index Scan scans each and every record in the index. Table Scan is where the table is processed row by row from beginning to end. If the index is a clustered index then an index scan is really a table scan. Since a scan touches every row in the table whether or not it qualifies, the cost is proportional to the total number of rows in the table. Hence, a scan is an efficient strategy only if the table is small. 

Index Seek:
Since a seek only touches rows that qualify and pages that contain these qualifying rows, the cost is proportional to the number of qualifying rows and pages rather than to the total number of rows in the table. 

Example
I have an employee table as shown in the diagram below. EmployeeId is the primary key. We have a clustered index on the employeeId column. 



Query 1 : Select * from Employee where FirstName='Ben'
Query 2 : Select * from Employee where EmployeeId=2

Query 1 will do an Index scan (Table Scan) to retrieve the record as there is no Index on the FirstName column. 
Query 2 will do an Index seek to retrieve the record as there is an Index on the EmployeeId column. 

So from this example, you should have understood that, a query will result into an index seek, only if there is an index on the table to help they query to retrieve the data.

Self Join with an example

There are 3 different types of joins available in sql server, and they are

1. Cross Join 
2. Inner Join or Join 
3. Outer Join

Outer Join is again divided into 3 types as shown below.
1. Left Outer Join or Left Join 
2. Right Outer Join or Right Join 
3. Full Outer Join or Full Join 

I strongly recomend to learn about the basics and types of joins, before reading this article. Read the articles below, before proceeding with self join.
1. Basics of Joins
2. Inner Join
3. Left Outer Join
4. Right Outer Join
5. Full Outer Join


Self join is not a different type of join. Self join means joining a table with itself. We can have an inner self join or outer self join. Let us try to understand with an example.

To set up the data for the example, use the script below to create Employee Table and populate it with some sample data. We will be usingEmployee Table to understand Self Join.

CREATE TABLE EMPLOYEE
(
[EMPLOYEEID] INT PRIMARY KEY,
[NAME] NVARCHAR(50),
[MANAGERID] INT
)
GO

INSERT INTO EMPLOYEE VALUES(101,'Mary',102)
INSERT INTO EMPLOYEE VALUES(102,'Ravi',NULL)
INSERT INTO EMPLOYEE VALUES(103,'Raj',102)
INSERT INTO EMPLOYEE VALUES(104,'Pete',103)
INSERT INTO EMPLOYEE VALUES(105,'Prasad',103)
INSERT INTO EMPLOYEE VALUES(106,'Ben',103)
GO

We use Self Join, if we have a table that references itself. For example, In the Employee Table below MANAGERID column references EMPLOYEEID column. So the table is said to referencing itself. This is the right scenario where we can use Self Join. Now I want to write a query that will give me the list of all Employee Names and their respective Manager Names. In order to achieve this I can use Self Join. In the Table below,Raj is the manager for Pete,Prasad and Ben. Ravi is the manager for Raj and Mary. Ravi does not have a manager as he is the president of the Company.

The query below is an example of Self Join. Both E1 and E2 refer to the same Employee Table. In this query we are joining the Employee Table with itself. 

SELECT E1.[NAME],E2.[NAME] AS [MANAGER NAME]
FROM EMPLOYEE E1 
INNER JOIN EMPLOYEE E2 
ON E2.EMPLOYEEID =E1.MANAGERID

If we run the above query we only get 5 rows out of the 6 rows as shown below.

Inner Self Join

This is because Ravi does not have a Manager. MANAGERID column for Ravi is NULL. If we want to get all the rows then we can use LEFT OUTER JOIN as shown below.

SELECT E1.[NAME],E2.[NAME] AS [MANAGER NAME]
FROM EMPLOYEE E1 
LEFT OUTER JOIN EMPLOYEE E2 
ON E2.EMPLOYEEID =E1.MANAGERID

If we execute the above query we get all the rows, including the row that has a null value in the MANAGERID column. The results are shown below. The MANAGERNAME for 2nd record is NULL as Ravi does not have a Manager.

Left Outer Self Join

Let us now slightly modify the above query using COALESCE as shown below. Read COALESCE function in SQL Server to understand COALESCE in a greater detail.

SELECT E1.[NAME],COALESCE(E2.[NAME],'No Manager') AS [MANAGER NAME]
FROM EMPLOYEE E1 
LEFT JOIN EMPLOYEE E2 
ON E2.EMPLOYEEID =E1.MANAGERID

If we execute the above query the output will be as shown in the image below. This is how COALESCE can be used.

Left Outer Self Join with COALESCE

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.

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.

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 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
RIGHT 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 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.