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Tuesday, February 14, 2012

General Questions on SQL SERVER

DBMS – Data Base Management System
RDBMS – Relational Data Base Management System or Relational DBMS
A DBMS has to be persistent, that is it should be accessible when the program created the data ceases to exist or even the application that created the data restarted. A DBMS also has to provide some uniform methods independent of a specific application for accessing the information that is stored.
RDBMS adds the additional condition that the system supports a tabular structure for the data, with enforced relationships between the tables. This excludes the databases that don’t support a tabular structure or don’t enforce relationships between tables.
DBMS does not impose any constraints or security with regard to data manipulation it is user or the programmer responsibility to ensure the ACID PROPERTY of the database whereas the RDBMS is more with this regards because RDBMS defines the integrity constraint for the purpose of holding ACID PROPERTY. RDBMS may be or may not be Client Server Database System.
Examples :
DBMS – File System, XML
RDBMS – SQL Server, Oracle
Reference : Pinal Dave (http://blog.SQLAuthority.com)

What are the Properties of the Relational Tables?

Relational tables have the following six properties:
  • Values are atomic.
  • Column values are of the same kind.
  • Each row is unique.
  • The sequence of columns is insignificant.
  • The sequence of rows is insignificant.
  • Each column must have a unique name.

What is Normalization?

Database normalization is a data design and organization process applied to data structures based on rules that help building relational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.

What is De-normalization?

De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.

How is ACID property related to Database?

ACID (an acronym for Atomicity Consistency Isolation Durability) is a concept that Database Professionals generally look for while evaluating databases and application architectures. For a reliable database, all this four attributes should be achieved.
Atomicity is an all-or-none proposition.
Consistency guarantees that a transaction never leaves your database in a half-finished state.
Isolation keeps transactions separated from each other until they are finished.

What are the Different Normalization Forms?

1NF: Eliminate Repeating Groups
Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
If an attribute depends on only part of a multi-valued key, then remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
Third Normal Form (3NF) is most preferable normal form in RDBMS. Normalization is the process of designing a data model to efficiently store data in a database. The rules of 3NF are mentioned here
  • Make a separate table for each set of related attributes, and give each table a primary key.
  • If an attribute depends on only part of a multi-valued key, remove it to a separate table
  • If attributes do not contribute to a description of the key, remove them to a separate table.
Normalization is very close to concept of object oriented schema’s and it stores one data at only one place by removing all the redundant data. It also helps to draw the schema easier. Normalization comes at the cost of performance.
BCNF: Boyce-Codd Normal Form
If there are non-trivial dependencies between candidate key attributes, then separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
5NF: Isolate Semantically Related Multiple Relationships
There may be practical constrains on information that justify separating logically related many-to-many relationships.
ONF: Optimal Normal Form
A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.
DKNF: Domain-Key Normal Form
A model free from all modification anomalies is said to be in DKNF.
Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.

Reference: Pinal Dave (http://blog.SQLAuthority.com)

 


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