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  • Difficulty Level : Medium
  • Last Updated : 30 Sep, 2021

Structured Query Language (SQL) is a nonprocedural language used for retrieving data from the queries. It was introduced by IBM as a part of the R project. It was declared as a standard language by ANSI and ISO. DQL statements are used for performing queries on the data within schema objects. The purpose of the DQL Command is to get some schema relation based on the query passed to it. We can define DQL as follows it is a component of SQL statement that allows getting data from the database and imposing order upon it. It includes the SELECT statement. This command allows getting the data out of the database to perform operations with it. When a SELECT is fired against a table or tables the result is compiled into a further temporary table, which is displayed or perhaps received by the program i.e. a front-end. Data Query Language (DQL) or Data Retrieval Language (DRL). Data Query Language has commands that retrieve the data from the query. It has a single command: select.  It has subcategories: 

  • DDL or Data Definition Language has commands like create, rename, alter. 
     
  • DML or Data Manipulation Language has commands like update, insert, delete. 
     
  • DCL or Data Control Language has commands like grant and revoke. 
     
  • TCL or Transaction Control Language has commands like rollback, commit. 
     

Characteristics of DQL: 
 

It is used for retrieving data. A user can retrieve data according to his/her requirements using the select command. 

Example: 
If a user wants to retrieve all the from the employee table, the query must be given as follows: 
 

select *
from employee;  

asterisk mark (*) indicates that all the rows are retrieved from the table. 

If a user wants to know the average salary of the employees, the query must be given as follows: 
 

select avg(salary) 
from employee; 

This displays the average salary of the employees. 
Here asterisk mark isn’t needed as we need to retrieve only particular rows. 

If a user wants to know the name of the employee whose salary is less than 10, 000; the query is: 
 

select ename 
from employee 
where salary<=10, 000; 

A condition is placed to retrieve the data and a relational operator is used as well. 

Advantages: 

  • SELECT is a standard command used for almost every query.
  • It can also be written as SELECT as SQL is a case-insensitive language.
  • It makes the data retrieving process easier.
  • No coding is needed.
  • This domain language can be used for communicating with the databases and receive answers to complex questions in seconds.

Disadvantages:

  • Data Query Language has no disadvantages.
  • It is not possible to imagine Structured Query Language without the select command.
  • Interfacing a DQL database is more complex than adding a few lines of code.
  • The operating cost of some DQL versions makes it difficult for some programmers to access them.

Applications: 

  • Analytical Queries.
  • Retrieve Information from the database.
  • Modify the index structures and database table.
     
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