Database Languages

A database system provides a data-definition language to specify the database schema and a data-manipulation language to express database queries and up dates. 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.

1.4.1 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 access are:
• 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.
  
• DeclarativeDMLs(alsoreferredtoasnonproceduralDMLs)requireauserto 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. We study the most widely used query language, SQL, in Chap- ters 3, 4, and 5. We also study some other query languages in Chapter 6.
  The levels of abstraction that we discussed in Section 1.3 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 (which we study in Chapters 12 and 13) translates DML queries into sequences of actions at the physical level of the database system.
  

1.4.2 Data-Definition Language
We specify a database schema by a set of definitions expressed by a special language called a data-definition language (DDL). The DDL is also used to specify additional properties of the data.