What is
RDBMS?
RDBMS
stands for Relational Database Management System.
RDBMS is the basis for SQL, and for all modern database systems like MS SQL
Server, IBM DB2, Oracle, MySQL, and Microsoft Access.
A
Relational database management system (RDBMS) is a database management system
(DBMS) that is based on the relational model as introduced by E. F. Codd.
What is
table ?
The data in
RDBMS is stored in database objects called tables. The table is a
collection of related data entries and it consists of columns and rows.
Remember, a table is the most common and
simplest form of data storage in a relational database.
What is
field?
Every table
is broken up into smaller entities called fields.
A field is
a column in a table that is designed to maintain specific information about
every record in the table.
What is
record, or row?
A record, also called a row of data, is each
individual entry that exists in a table.
A record is
a horizontal entity in a table.
What is
column?
A column is
a vertical entity in a table that contains all information associated with a
specific field in a table.
What is
NULL value?
A NULL
value in a table is a value in a field that appears to be blank which means A
field with a NULL value is a field with no value.
It is very
important to understand that a NULL value is different than a zero value or a
field that contains spaces. A field with a NULL value is one that has been left
blank during record creation.
SQL
Constraints:
Constraints
are the rules enforced on data columns on table. These are used to limit the
type of data that can go into a table. This ensures the accuracy and reliability
of the data in the database.
Contraints
could be column level or table level. Column level constraints are applied only
to one column where as table level constraints are applied to the whole table.
Following
are commonly used constraints available in SQL:
- NOT NULL
Constraint : Ensures that a column cannot have NULL value.This Ensure that at least some value should be present for an attribute . Can have more then one NOT NULL constraints on a table
- DEFAULT
Constraint : Provides a default value for a column when none is
specified.
- UNIQUE Constraint:
Ensures that all values in a column are different.It check for Duplicate values.A UNIQUE column can have multiple null value .
- PRIMARY Key:
Uniquely identified each rows/records in a database table.It is used for identifying a record uniquely. It is combination of NOT NULL and UNIQUE. a) CANDIDATE KEYS : The columns that are eligible to become PK are called candidate keys. b) ALTERNATE KEY: A column which is eligible to become PK but not chosen as PK is called Alternate key. c) COMPOSITE PK: When PK is created out of more then one column then is called composite.
- FOREIGN Key:
Uniquely identified a rows/records in any another database table.foreign key is always created on the child tables.
- CHECK Constraint:
The CHECK constraint ensures that all values in a column satisfy certain
conditions.
- INDEX: Use to
create and retrieve data from the database very quickly.
Data
Integrity:
The
following categories of the data integrity exist with each RDBMS:
- Entity Integrity : There are no duplicate
rows in a table.
- Domain Integrity : Enforces valid entries
for a given column by restricting the type, the format, or the range of
values.
- Referential integrity : Rows cannot be deleted,
which are used by other records.
- User-Defined Integrity : Enforces some specific
business rules that do not fall into entity, domain, or referential
integrity.
Database
Normalization
Database
normalization is the process of efficiently organizing data in a database.
There are two reasons of the normalization process:
- Eliminating redundant data, for
example, storing the same data in more than one tables.
- Ensuring data dependencies make
sense.
Both of these
are worthy goals as they reduce the amount of space a database consumes and
ensure that data is logically stored. Normalization consists of a series of
guidelines that help guide you in creating a good database structure.
Normalization
guidelines are divided into normal forms; think of form as the format or the
way a database structure is laid out. The aim of normal forms is to organize
the database structure so that it complies with the rules of first normal form,
then second normal form, and finally third normal form.
It's your
choice to take it further and go to fourth normal form, fifth normal form, and
so on, but generally speaking, third normal form is enough.
- First
Normal Form (1NF)
- Second
Normal Form (2NF)
- Third
Normal Form (3NF)
