DQL Full Form

Structured Query Language (SQL) is a non procedural language used for retrieving data from the queries. It was introduced by IBM as a part of R project. It was declared as a standard language by ANSI and ISO.

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.

Data Manipulation Language has a subcategory:

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.

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



Example:
If a user want 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 is retrieved from the table.

If a user want 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 more easier.

Disadvantages:

  • Data Query Language has no disadvantages.
  • It is not possible to imagine Structured Query Language without the select command.

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