A database system provides a data-definition language to specify the database
schema and a data-manipulation language to express database queries and updates.
In practice, the data-definition and data-manipulation languages are not
two separate languages; instead they simply form parts of a single database language,
such as the widely used SQL language:
1. Data-Manipulation Language
A data-manipulation language (DML) is a language that enables users to access
or manipulate data as organized by the appropriate data model. The types of
• Retrieval of information stored in the database
• Insertion of new information into the database
• Deletion of information from the database
• Modification of information stored in the database
There are basically two types:
• Procedural DMLs require a user to specify what data are needed and how to
get those data.
• Declarative DMLs (also referred to as nonprocedural DMLs) require a user to
specify what data are needed without specifying how to get those data.
Declarative DMLs are usually easier to learn and use than are procedural
DMLs. However, since a user does not have to specify how to get the data, the
database system has to figure out an efficient means of accessing data.
A query is a statement requesting the retrieval of information. The portion of
a DML that involves information retrieval is called a query language. Although
technically incorrect, it is common practice to use the terms query language and
data-manipulation language synonymously.
There are a number of database query languages in use, either commercially
or experimentally. The levels of abstraction apply not only to defining or structuring
data, but also to manipulating data. At the physical level, we must define algorithms
that allow efficient acccess to data. at higher levels of abstraction, we emphasize ease of
use. the goal is to allow humans to interact efficiently with the system. The query
processor component of the database system translates DML queries
into sequences of actions at the physical level of the database system.
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