Self - Join in SQL Server

A self-join is simply a normal SQL join that joins one table to itself. 
This is accomplished by using table name aliases to give each instance of the table a separate name.

When Useful?Joining a table to itself can be useful when you want to compare values in a column to 
other values in the same column.

We have sample records in Employee table.


Inner Join Example----

SELECT e1.Name EmployeeName, e2.name AS ManagerNameFROM Employee e1
INNER JOIN Employee e2ON e1.ManagerID = e2.EmployeeID

Outer Join example---

SELECT e1.Name EmployeeName, ISNULL(e2.name, 'Top Manager') ASManagerNameFROM Employee e1
LEFT JOIN Employee e2ON e1.ManagerID = e2.EmployeeID

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Joins Examples

I will explain types of SQL JOINs in in this article. JOINs in SQL Server can be classified as follows:

• INNER JOIN
• LEFT OUTER JOIN
• RIGHT OUTER JOIN
• FULL OUTER JOIN
• CROSS JOIN

Each type is explained below with suitable examples:
Let’s consider two tables as Table1 & Table2 for our example.

– CREATE TEST TABLES
CREATE TABLE Table1(ID [int], Code [varchar](10));
GO
CREATE TABLE Table2(ID [int], Amount [int]);
GO


– INSERT DUMMY DATA
INSERT INTO Table1 VALUES
(1,’AAA’),(2,’BBB’),(3,’CCC’)
GO
INSERT INTO Table2 VALUES
(1,50),(2,30),(4,40)
GO

INNER JOIN:
Inner joins return rows only when there is at least one row from both tables that matches the join condition. Inner joins eliminate the rows that do not match with a row from the other table. Below is the query for inner join:

SELECT *
FROM Table1 A
INNER JOIN Table2 B
   ON A.ID = B.ID
GO

LEFT OUTER JOIN
Left Outer joins return all rows from the left table referenced with a left outer join and matching rows from other table. Unmatched records will be NULL. Below is the query for Left Outer Join:

SELECT *
FROM Table1 A
LEFT OUTER JOIN Table2 B
   ON A.ID = B.ID
GO

RIGHT OUTER JOIN
Right Outer joins return all rows from the right table referenced with a right outer join and matching rows from other table. Unmatched records will be NULL. Below is the query for Right Outer Join:

SELECT *
FROM Table1 A
RIGHT OUTER JOIN Table2 B
ON A.ID = B.ID
GO

FULL OUTER JOIN
Full Outer joins return all rows from both the tables. Unmatched records will be NULL. Below is the query for Full Outer Join:

SELECT * FROM Table1 A
FULL OUTER JOIN Table2 B
ON A.ID = B.ID
GO

CROSS JOIN
In cross joins, each row from first table joins with all the rows of another table. If there are m rows from Table1 and n rows from Table2 then result set of these tables will have m*n rows. Below is the query for 

SELECT * FROM Table1 A
CROSS JOIN Table2 B
GO

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Advantages of Views

1) What is a View in SQL Server?
You can think of a view either as a compiled sql query or a virtual table. As a view represents a virtual table, it does not physically store any data. When you query a view, you actually retrieve the data from the underlying base tables.2) What are the advantages of using views?
1. Views can be used to implement row level and column level security.Example 1: Consider the tblEmployee table below. I don't want some of the users to have access to the salary column, but they should still be able to access ID, NAME and DEPT columns. If I grant access to the table, the users will be able to see all the columns. So, to achieve this, I can create a view as shown in Listing 1 below. Now, grant access to the view and not the table. So using views we can provide column level security.

tblEmployee Listing 1 Create View      vWEmployee As Select               ID, Name, Dept From                 tblEmployee

Example 2: Let us say, we have a few users who should be able to access only IT employee details and not any other dept. To do this, I can create a view as shown in Listing 2 below. Now, grant access only to the view and not the table. So using views we can provide row level security as well.Listing 2
Create View     vWITEmployees
As
Select              ID, Name, Dept
From                tblEmployee
Where              Dept = 'IT'2. Simplify the database schema to the users. You can create a view based on multiple tables which join columns from all these multiple tables so that they look like a single table.
3. Views can be used to present aggregated and summarized data.
Example 1: Consider the tblEmployee table above. I want to aggregate the data as shown in the image below. To do this I can create a view as shown in Listing 3. Now, you can simply issue a select query against the view rather than writing a complex query every time you want to retrieve the aggregated data.

Listing 3 Select        Dept, Count(*) As Total From          tblEmployee Group By   Dept

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SQL Index Advantages and Disadvantages

1) What are the disadvantages of an Index?
There are 2 disadvantages of an Index
1. Increased Disk Space
2. Insert, Update and Delete statements could be slow. In short, all DML statements could be slow.
Disk Space: Indexes are stored on the disk, and the amount of space required will depend on the size of the table, and the number and types of columns used in the index. Disk space is generally cheap enough to trade for application performance, particularly when a database serves a large number of users. 
Insert, Update and Delete statements could be slow: Another downside to using an index is the performance implication on data modification statements. Any time a query modifies the data in a table (INSERT, UPDATE, or DELETE), the database needs to update all of the indexes where data has changed. Indexing can help the database during data modification statements by allowing the database to quickly locate the records to modify, however, providing too many indexes to update can actually hurt the performance of data modifications. This leads to a delicate balancing act when tuning the database for performance.

2)What are the 2 types of Indexes in SQL Server?
1. Clustered Index 
2. Non Clustered Index

How many Clustered and Non Clustered Indexes can you have per table?
Clustered Index - Only one Clustered Index per table. A clustered index contains all of the data for a table in the index, sorted by the index key. Phone Book is an example for Clustered Index.
Non Clustered Index - You can have multiple Non Clustered Indexes per table. Index at the back of a book is an example for Non Clustered Index.

3)Which Index is faster, Clustered or Non Clustered Index?
Clustered Index is slightly faster than Non Clustered Index. This is because, when a Non Clustered Index is used there is an extra look up from the Non Clustered Index to the table, to fetch the actual rows.

4)When is it usually better to create a unique nonclustered index on the primary key column?
Sometimes it is better to use a unique nonclustered index on the primary key column, and place the clustered index on a column used by more queries. For example, if the majority of searches are for the price of a product instead of the primary key of a product, the clustered index could be more effective if used on the price field.

5)What is a Composite Index in SQL Server?
or 
What is the advantage of using a Composite Index in SQL Server?
or 
What is Covering Query? A composite index is an index on two or more columns. Both clustered and nonclustered indexes can be composite indexes.
If all of the information for a query can be retrieved from an Index. A clustered index, if selected for use by the query optimizer, always covers a query, since it contains all of the data in a table.

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