Some essentials in SQL Server...

What is normalization? what are different types of normalization?
It is set of rules that have been established to aid in the design of tables that are meant to be connected through relationships. This set of rules is known as normalization.
Benefits of normalizing your database will include :
Avoiding repetitive entries
Reducing required storage space
Preventing the need to restructure existing tables to accommodate new data
Increased speed and flexibility of queries, sorts, and summaries.
Following are the three normal forms :
First Normal Form
For a table to be in first normal form, data must be broken up into the smallest units possible. In addition to breaking data up into the smallest meaningful values, tables in first normal form should not contain repetitions groups of fields.
Second Normal Form
The second normal form states that each field in a multiple field primary key table must be directly related to the entire primary key. Or in other words, each non-key field should be a fact about all the fields in the primary key.
Third Normal Form
A non-key field should not depend on other Non-key field.

What is denormalization?
Denormalization is the process of putting one fact in numerous places (its vice-versa of normalization).Only one valid reason exists for denormalizing a relational design - to enhance performance or if we are doing data warehousing and data mining. The sacrifice to performance is that you increase redundancy in database.

What is a candidate key?
A table may have more than one combination of columns that could uniquely identify the rows in a table; each combination is a candidate key. During database design you can pick up one of the candidate keys to be the primary key.

What are the different types of joins? What is the difference between them?
Inner Join
Inner join shows matches only when they exist in both tables.
Example is given below
SELECT Customers.*, Orders.* FROM Customers INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
Left Outer Join
Left join will display all records in left table of the SQL statement. In SQL below customers with or without orders will be displayed.
Example is given below
SELECT Customers.*, Orders.* FROM Customers LEFT OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
Right Outer Join
Right join will display all records in right table of the SQL statement. In SQL below all orders with or without matching customer records will be displayed.
Example is given below
SELECT Customers.*, Orders.* FROM Customers RIGHT OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID

What are indexes? what is the difference between clustered and nonclustered indexes?
Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quickly.
There are clustered and nonclustered indexes. A clustered index is a special type of index that reorders the way in which records in the table are physically stored. Therefore, table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
A nonclustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk.The leaf node of a nonclustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.