SQL Overview

SQL (pronounced “ess-que-el”) stands for Structured Query Language. SQL is used to communicate with a database. According to ANSI (American National Standards Institute), it is the standard language for relational database management systems.It is designed over relational algebra and tuple relational calculus. SQL comes as a package with all major distributions of RDBMS.


Data definition Language

SQL uses the following set of commands to define database schema:

CREATE

Creates new databases, tables and views from RDBMS

For example:

Create database shiksha360;
Create table article;
Create view for_students;

DROP

Drop commands deletes views, tables and databases from RDBMS

Drop object_type object_name;
Drop database shiksha360;
Drop table article;
Drop view for_students;

ALTER

Modifies database schema.

Alter object_type object_name parameters;

for example:

Alter table article add subject varchar;

This command adds an attribute in relation article with name subject of string type.

Data Manipulation Language

SQL is equipped with data manipulation language. DML modifies the database instance by inserting, updating and deleting its data. DML is responsible for all data modification in databases. SQL contains the following set of command in DML section:

  • SELECT/FROM/WHERE

  • INSERT INTO/VALUES

  • UPDATE/SET/WHERE

  • DELETE FROM/WHERE

These basic constructs allows database programmers and users to enter data and information into the database and retrieve efficiently using a number of filter options.

SELECT/FROM/WHERE

  • SELECT

    This is one of the fundamental query command of SQL. It is similar to projection operation of relational algebra. It selects the attributes based on the condition described by WHERE clause.

  • FROM

    This clause takes a relation name as an argument from which attributes are to be selected/projected. In case more than one relation names are given this clause corresponds to Cartesian product.

  • WHERE

    This clause defines predicate or conditions which must match in order to qualify the attributes to be projected.

    For example:

    Select author From article Where age > 50;
    

    This command will project names of author’s from book_author relation whose age is greater than 50.

INSERT INTO/VALUES

This command is used for inserting values into rows of table (relation).

Syntax is

INSERT INTO table (column1 [column2, column3 ... ]) VALUES (value1 [value2, value3 ... ])

Or

INSERT INTO table VALUES (value1, [value2, ... ])

For Example:

INSERT INTO article(Author, Subject) VALUES ("anonymous", "computers");

UPDATE/SET/WHERE

This command is used for updating or modifying values of columns of table (relation).

Syntax is

UPDATE table_name SET column_name = value [column_name = value ...] [WHERE condition]

For example:

UPDATE article SET Author="webmaster" WHERE Author="anonymous";

DELETE/FROM/WHERE

This command is used for removing one or more rows from table (relation).

Syntax is

DELETE FROM table_name [WHERE condition];

For example:

DELETE FROM article WHERE Author="unknown";

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