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. Following is the
example of a CUSTOMERS table:
+----+----------+-----+-----------+----------+
|
ID | NAME
| AGE |
ADDRESS |
SALARY |
+----+----------+-----+-----------+----------+
| 2
|
Khilan | 25
|
Delhi | 1500.00
|
| 3
|
kaushik | 23
|
Kota | 2000.00
|
| 4
|
Chaitali
| 25
|
Mumbai | 6500.00
|
| 5
|
Hardik | 27
|
Bhopal | 8500.00
|
| 6
|
Komal | 22
|
MP | 4500.00
|
| 7
|
Muffy | 24
|
Indore |
10000.00
|| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
+----+----------+-----+-----------+----------+
What is field?
Every table is broken up into smaller
entities called fields. The fields in the CUSTOMERS table consist of ID, NAME,
AGE, ADDRESS and SALARY.
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. For example there are 7 records
in the above CUSTOMERS table. Following is a single row of data or record in
the CUSTOMERS table:
+----+----------+-----+-----------+----------+
| 1
|
Ramesh | 32
|
Ahmedabad
| 2000.00
|
+----+----------+-----+-----------+----------+
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.
For example, a column in the CUSTOMERS
table is ADDRESS which represents location description and would consist of the
following:
+-----------+
|
ADDRESS |
+-----------+
|
Ahmedabad
|
|
Delhi |
|
Kota |
|
Mumbai |
|
Bhopal |
|
MP |
|
Indore |
+----+------+
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:
·
Data Integrity:
The following categories of the data
integrity exist with each RDBMS:
·
NOT NULL Constraint: Ensures that a column
cannot have NULL value.
·
DEFAULT Constraint : Provides a default
value for a column when none is specified.
·
UNIQUE Constraint: Ensures that all
values in a column are different.
·
PRIMARY Key: Uniquely identified
each rows/records in a database table.
·
FOREIGN Key: Uniquely identified a
rows/records in any another database table.
·
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
·
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.
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