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Data manipulation is

The retrieval of information stored in the database.

The insertion of new information into the database.

The deletion of information from the database.

The modification of information stored in the database.  

A data-manipulation language (DML) is a language that enables users to access or manipulate data as organized by the appropriate data model. 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. The DML component of the SQL language is nonprocedural.  

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.  

This query in the SQL language finds the name of the customer whose customer-id is 192-83-7465:  

select customer.customer-name from customer where customer.customer-id = 192-83-7465

The query specifies that those rows from the table customer where the customer-id is 192-83-7465 must be retrieved, and the customer-name attribute of these rows must be displayed.  

Queries may involve information from more than one table. For instance, the following query finds the balance of all accounts owned by the customer with customerid 192-83- 7465.  

select account.balance from depositor, account where depositor.customer-id = 192-83-7465 and depositor.account-number = account.account-number  

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 access 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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