The DBMS provides different languages and interfaces for each category of users to express database queries and updations. When the design of the database is complete and the DBMS is chosen to implement it, the first thing to be done is to specify the conceptual and internal schemas for the database and the corresponding mappings. The following five languages are available to specify different schemas.
- Data Definition Language (DDL)
- Storage Definition Language (SDL)
- View Definition Language (VDL)
- Data Manipulation Language (DML)
- Fourth-Generation Language (4-GL)
1. Data Definition Language (DDL)
It is used to specify a database conceptual schema using set of definitions. It supports the definition or declaration of database objects. Many techniques are available for writing DDL. One widely used technique is writing DDL into a text file. More about DDL in chapter 7.
2. Storage Definition Language (SDL)
It is used to specify the internal schema in the database. The storage structure and access methods used by the database system is specified by the specified set of SDL statements. The implementation details of the database schemas are implemented by the specified SDL statements and are usually hidden from the users.
3. View Definition Language (VDL)
It is used to specify user’s views and their mappings to the conceptual schema. But generally, DDL is used to specify both conceptual and external schemas in many DBMS’s. There are two views of data the logical view—that is perceived by the programmer and physical view—data stored on storage devices.
4. Data Manipulation Language (DML)
It provides a set of operations to support the basic data manipulation operations on the data held in the database. It is used to query, update or retrieve data stored in a database. The part of DML that provide data retrieval is called query language.
The DML is of the two types :
- Procedural DML : It allows the user to tell the system what data is needed and how to retrieve
- Non-procedural DML : It allows the user to state what data are needed, rather than how it is to be More about DML in chapter 7.
5. Fourth-Generation Language (4-GL)
It is a compact, efficient and non-procedural programming language used to improve the efficiency and productivity of the DBMS. In this, the user defines what is to be done and not how it is to be done. The 4-GL has the following components in it. These are:
(a) Query languages (b) Report
(c) Spread sheets (d) Database languages
(e) Application generators
(f ) High level languages to generate application program.
System Query Language (SQL) is an example of 4-GL. More about SQL in Chapter 7.
Source: Gupta Satinder Bal, Mittal Aditya (2017), Introduction to Database Management System, 2nd Edition-University Science Press (2017)