What is meant by Normalization and De-Normalization
It is the process of efficiently organizing data in a database. There are two goals of the normalization process:
- Eliminate redundant data (for example, storing the same data in more than one table) and increase consistency in the database.
- Ensure data dependencies make sense (only storing related data in a table). Both of these are worthy goals as they reduce the amount of space a database consumes and ensure that data is logically grouped and stored.
Following are the key steps for Normalization process
Eliminate Repeating Groups – Make a separate table for each set of related attributes, and give each table a primary key.
Eliminate Redundant Data - If an attribute depends on only part of a multi-valued key, remove it to a separate table.
Eliminate columns Not Dependent On Key – If attributes do not contribute to a description of the key, remove them to a separate table.
Isolate Independent Multiple Relationships - No table may contain two or more 1:n or n:m relationships that are not directly related.
Isolate Semantically Related Multiple Relationships - There may be practical constrains on information that justify separating logically related many-to-many relationships.
Different Types of Normal Forms
1st Normal Form (1NF):
Def: A table (relation) is in 1NF if
1. There are no duplicated rows in the table.
2. Each cell is single-valued (i.e., there are no repeating groups or arrays).
3. Entries in a column (attribute, field) are of the same kind.
Note:
- The order of the rows and columns are immaterial(of no importance).
- The requirement that there be no duplicated rows in the table means that the table has a key (although the key might be made up of more than one column—even, possibly, of all the columns).
2nd Normal Form (2NF):
Def: A table is in 2NF if it is in 1NF and if all non-key attributes are dependent on all of the key.
Note: Since a partial
dependency occurs when a non-key attribute is dependent on only a part
of the (composite) key, the definition of 2NF is sometimes phrased as,
“A table is in 2NF if it is in 1NF and if it has no partial
dependencies.”
3rd Normal Form (3NF):
Def: A table is in 3NF if it is in 2NF and if it has no transitive dependencies.
Note: A transitive dependency
is a type of functional dependency in which the value in a non-key field
is determined by the value in another non-key field and that field is
not a candidate key.
Boyce-Codd Normal Form (BCNF):
Def: A table is in BCNF if it is in 3NF and if every determinant is a candidate key.
4th Normal Form (4NF):
Def: A table is in 4NF if it is in BCNF and if it has no multi-valued dependencies. A multi-valued dependency
occurs when, for each value in field A, there is a set of values for
field B and a set of values for field C but fields B and C are not
related.
5th Normal Form (5NF):
Def: A table is in 5NF, also called
“Projection-Join Normal Form” (PJNF), if it is in 4NF and if every join
dependency in the table is a consequence of the candidate keys of the
table.
Domain-Key Normal Form (DKNF):
Def: A table is in DKNF if every constraint on the table is a logical consequence of the definition of keys and domains.
De-Normalization
Denormalization is a technique to move
from higher to lower normal forms of database modeling in order to speed
up database access. You may apply Denormalization in the process of
deriving a physical data model from a logical form.
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