select employee_id, count(*) from employees group by
employee_id having count(*) > 1

Normalization is a design technique to develop table 
structures. Purpose of normalization is to eliminate 
redundant data.

Normalization is a technique by which successive redundancy 
of a given relation is make into a more easier form.it 
improves the tuple quality and make the data in a 
consistence state,also helps to improves redundant data.
(Higher the normal form better the tuple type)

Float
Binary Data

Bit datatype is used to store boolean information like 1 or 
0 (true or false). Untill SQL Server 6.5 bit datatype could 
hold either a 1 or 0 and there was no support for NULL. But 
from SQL Server 7.0 onwards, bit datatype can represent a 
third state, which is NULL.

Defaults are used to assign some defalut values to 
columns . These values are used if user doen to specify a 
column value.
Timestamp canot have default clause attached to it

A composite Key can be either Primay or Unique Key
More then One Key columns are said to be composite keys

Candidate Key(Primary Key) is a Key which Maintains the Row 
Uniue .Can be defined based on the Entity


Alternate Key or Unique Key is similar to PK ,except it 
accepts null Values .So that the records can still be 
entered submitting null values to this attribute.

primary key:-  The attribute or combination of attributes
that uniquely identifies a row or record.

Foreign Key:- an attribute or combination of attribute in a
table whose value match a primary key in another table.

Composite key:- A primary key that consistsof two or more
attributes is known as composite key

candidate key:-  is a column in a table which has the
ability to become a primary key.

Alternate Key:- Any of the candidate keys that is not part
of the primary key is called an alternate key.

candidate key:-A candidate key uniquely identifies the rows
in a table.there can be more than 1 candidate key in a 
table, in which one should be a primary key.

alternate key:-In the candidate key the key which is not 
primary key is the alternate key.

composite key:- A key formed bythe combination of 2 or more 
columns is the composite key.

A candidate key is able to identify a unique tuple in a 
relation through an attribute or attribute set.

A foreign key makes sure that a tuple in a relation does 
not match a tuple in another relation which does not exist

Candidate Key.....???? Help me out..!!!!!

A candidate key is one that can identify each row
of a table uniquely. Generally a candidate key becomes the
primary key of the table. If the table has more than one
candidate key, one of them will become the primary key, and
the rest are called alternate keys.
           A key formed by combining at least two or more
columns is called composite key.

One to one 
Parent Entity does have one Corresponding record in Child 
Entity

ex : Employee and his CV

One to many
 
Parent Entity does have many Corresponding records in Child 
Entity

ex : Employee and his Projects

many to many
 
Parent Entity records do  have many Corresponding records in 
Child Entity vice versa

ex : Employees and Employers 
Employees Receive Calls from Many Employers 
Employers Receive CVs from Many Employees

Denormalization is introcution of controlled redundancy in 
the database. 

Use denormalization generally to optimize performace of  
select query. to minimize joins used in the query.

Denormalization is a technique to move from higher to lower 
normal forms of database modeling in order to speed up 
database access

The task of a database designer is to structure the data in
a way that eliminates unnecessary duplication and provides a
rapid search path to all necessary information. The process
of refining tables, keys, columns, and relationships to
create an efficient database is called normalization

because there is no redundant, neglected data and normalized
tables produce rapid, efficient joins.

There are 5 different normalization methods.

 First Normal Form (1NF): Each field in a table must contain
different information.

 Second Normal Form (2NF): All attributes that are not
dependent upon the primary key in a database table must be
eliminated.

 Third Normal Form (3NF): No duplicate information is
permitted. So, for example, if two tables both require a
common field, this common field information should be
separated into a different table.

 There are 2 more normalization forms, fourth normal form
(4NF) and fifth normal form (5NF), but they are rarely used.
Normalization makes databases more efficient and easier to
maintain.

Normalization:
        Normalization is the process of designing a data
model to efficiently store data in a database. The end
result is that redundant data is eliminated, and only data
related to the attribute is stored within the table.

First normal form (1NF):
	An entity type is in 1NF when it contains no repeating
groups of data.

Second normal form (2NF):
	An entity type is in 2NF when it is in 1NF and when all of
its non-key attributes are fully dependent on its primary key. 

Third normal form (3NF):
	An entity type is in 3NF when it is in 2NF and when all of
its attributes are directly dependent on the primary key.