SQL (Structure Query Language)

What is SQL?

SQL stands for Structured Query Language. SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987.

SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in a relational database.

Applications of SQL

SQL is one of the most widely used query language over the databases. I'm going to list few of them here:

·        Allows users to access data in the relational database management systems.

·        Allows users to describe the data.

·        Allows users to define the data in a database and manipulate that data.

·        Allows to embed within other languages using SQL modules, libraries & pre-compilers.

·        Allows users to create and drop databases and tables.

·        Allows users to create view, stored procedure, functions in a database.

·        Allows users to set permissions on tables, procedures and views.

SQL Commands

1.   DDL - Data Definition Language

• CREATE
• ALTER
• DROP

2.     DML - Data Manipulation Language

·        SELECT

·        INSERT

·        UPDATE            

·        DELETE

3.     DCL - Data Control Language

·        GRANT

·        REVOKE

4.   TCL – Transaction Control Language

• COMMIT
• ROLLBACK
• SAVEPOINT

Data Integrity

The following categories of data integrity exist with each RDBMS −

• Entity Integrity − There are no duplicate rows in a table.

• Domain Integrity − Enforces valid entries for a given column by restricting the type, the format, or the range of values.

• Referential integrity − Rows cannot be deleted, which are used by other records.

• User-Defined Integrity − Enforces some specific business rules that do not fall into entity, domain or referential integrity.

SQL Constraints

Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database.

Constraints can either be column level or table level. Column level constraints are applied only to one column whereas, table level constraints are applied to the entire table.

Following are some of the most commonly used constraints available in SQL −

 

• NOT NULL Constraint − Ensures that a column cannot have a NULL value.

• DEFAULT Constraint − Provides a default value for a column when none is specified.

• UNIQUE Constraint − Ensures that all the values in a column are different.

• PRIMARY Key − Uniquely identifies each row/record in a database table.

• FOREIGN Key − Uniquely identifies a row/record in any another database table.

• CHECK Constraint − The CHECK constraint ensures that all values in a column satisfy certain conditions.