Database Reference
In-Depth Information
Here are some examples of changes in the logical layer that can be safely made thanks to
logical data independence:
Adding a new database object
Adding data items to an existing object
Any change where a view can be placed in the external model that replaces (and
processes the same as) the original object in the logical layer, such as combining or splitting
existing objects
2.7 Database Languages
Database languages and Interfaces deal with how a data models gets into a database and
how the information gets to the user. These deal with interaction of an application with a
database management system and user query to a database and view of result . The DBMS
interface will comprise a database sublanguage. A database sublanguage is a programming
language designed specifically for initiating DBMS functions.
2.7.1 Data Definition Languages (DDL)
Data Definition Languages are used by the DBA and database designers to specify the con-
ceptual schema of a database. In other words , it is used to describe data and data structures.
These are also used to define internal and external schemas (view).
Examples of DDL commands:
CREATE: To make a new database, table, index, or stored query. DROP: To destroy an
existing database, table, index, or view.
ALTER : To modify an existing database object.
2.7.2 Data Manipulation Language (DML)
Data Manipulation Language is used to specify database retrievals and updates.
Various operations like store, read, search, change, etc. are performed by DML. DML
commands can be embedded in a general purpose programming language such as
PASCAL, COBOL or PL/1. Stand alone DML commands can be applied directly (query
language).
Examples of DML Commands:
Select : To query data from tables in a database
Insert : To insert new row(s) into a database.
Update: To update row(s) in a table
Delete: To delete row(s) from a table.
