GK AND GS(Computer) : सामान्य ज्ञान(कम्प्यूटर/परिकलक)

SQL

(Structured Query Language)

SQL consists of a 

1.data definition language, 

2.data manipulation language, and 

3.data control language

Data definition language

 DDL: A data definition language or data description language (DDL) is a syntax similar to a computer programming language for defining data structures, especially database schemas.

CREATE statement

The CREATE command is used to establish a new database, table, index, or stored procedure.

CREATE TABLE [table name] ( [column definitions] ) [table parameters]

An example statement to create a table named employees with a few columns is:

CREATE TABLE employees (

    id            INTEGER       PRIMARY KEY,

    first_name    VARCHAR(50)   not null,

    last_name     VARCHAR(75)   not null,

    fname         VARCHAR(50)   not null,

    dateofbirth   DATE          not null

);

DROP statement

The DROP statement destroys an existing database, table, index, or view.

DROP objecttype objectname.

For example, the command to drop a table named employees is:

DROP TABLE employees;

ALTER statement

The ALTER statement modifies an existing database object.

ALTER objecttype objectname parameters.

For example, the command to add (then remove) a column named bubbles for an existing table named sink is:

ALTER TABLE sink ADD bubbles INTEGER;

ALTER TABLE sink DROP COLUMN bubbles;

RENAME statement

The RENAME statement is used to rename a database table.

RENAME TABLE old_name TO new_name;

TRUNCATE statement

The TRUNCATE statement is used to delete all data from a table. It's much faster than DELETE.

TRUNCATE TABLE table_name;

Data manipulation language

A data manipulation language (DML) is a family of syntax elements similar to a computer programming language used for selecting, inserting, deleting and updating data in a database. Performing read-only queries of data is sometimes also considered a component of DML.

1. SELECT ... FROM ... WHERE ...
2. INSERT INTO ... VALUES ...
3. UPDATE ... SET ... WHERE ...
4. DELETE FROM ... WHERE ...

SELECT Statement

The SELECT statement is used to select data from a database. The data returned is stored in a result table, called the result-set.

SELECT Syntax

SELECT column1, column2, ...
FROM table_name;

(Here, column1, column2, ... are the field names of the table you want to select data from.)

 If you want to select all the fields available in the table, use the following syntax:

SELECT * FROM table_name;

SELECT DISTINCT Statement

The SELECT DISTINCT statement is used to return only distinct (different) values.

Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values.

The SELECT DISTINCT statement is used to return only distinct (different) values.

SELECT DISTINCT Syntax

SELECT DISTINCT column1, column2, ...
FROM table_name;

The SQL WHERE Clause

The WHERE clause is used to filter records.

The WHERE clause is used to extract only those records that fulfill a specified condition.

WHERE Syntax

SELECT column1, column2, ...
FROM table_name
WHERE condition;

Data control language

A data control language (DCL) is a syntax similar to a computer programming language used to control access to data stored in a database (Authorization). In particular, it is a component of Structured Query Language (SQL).

Examples of DCL commands include:

1. GRANT to allow specified users to perform specified tasks.
2. REVOKE to cancel previously granted or denied permissions

 

QUESTION ANSWER

1. SQL stands for-   Structured Query Language
2. RDBMS stands for-  Relational Database Management System.
3. A table is a collection of-  related data entries and it consists of columns and rows.