SQL

Definition:

§  Structured Query Language (SQL) is a standard computer language for relational database management and data manipulation.

§  SQL is used to query, insert, update and modify data.

§  Most relational databases support SQL, which is an added benefit for database administrators (DBAs), as they are often required to support databases across several different platforms.

§  First developed in the early 1970s at IBM by Raymond Boyce and Donald Chamberlin, SQL was commercially released by Relational Software Inc. (now known as Oracle Corporation) in 1979.

§  The current standard SQL version is voluntary, vendor-compliant and monitored by the American National Standards Institute (ANSI).

§  Most major vendors also have proprietary versions that are incorporated and built on ANSI SQL, e.g., SQL*Plus (Oracle), and Transact-SQL (T-SQL) (Microsoft).

§  SQL comprises both data definition and data manipulation languages.

§  Using the data definition properties of SQL, one can design and modify database schema, whereas data manipulation properties allows SQL to store and retrieve data from database.

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 tutorialspoint;

Create table article;

Create view for_students;

DROP

·         Drops commands, views, tables, and databases from RDBMS.

For example-

Drop object_type object_name;

Drop database tutorialspoint;

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;

Data Manipulation Language:

SQL is equipped with data manipulation language (DML). DML modifies the database instance by inserting, updating and deleting its data. DML is responsible for all froms data modification in a database. SQL contains the following set of commands in its DML section –

§  SELECT/FROM/WHERE

§  INSERT INTO/VALUES

§  UPDATE/SET/WHERE

§  DELETE FROM/WHERE

These basic constructs allow 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 commands of SQL. It is similar to the 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_name

From book_author

Where age > 50;

This command will yield the names of authors from the relation book_author whose age is greater than 50.

INSERT INTO/VALUES

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

Syntax-

INTO table (column1 [, column2, column3 … ]) VALUES (value1 [, value2, value3 … ]) Or

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

For example –

INSERT INTO tutorialspoint (Author, Subject) VALUES (“anonymous”, “computers”);

UPDATE/SET/WHERE

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

Syntax –

UPDATE table_name SET column_name = value [, column_name = value …] [WHERE condition]

For example –

UPDATE tutorialspoint SET Author=”webmaster” WHERE Author=”anonymous”;

DELETE/FROM/WHERE

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

Syntax –

DELETE FROM table_name [WHERE condition];

For example –

DELETE FROM tutorialspoints

WHERE Author=”unknown”;

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