SQL - Overview

SQL tutorial gives unique learning on Structured Query Language and it helps to make practice on SQL commands which provides immediate results. SQL is a language of database, it includes database creation, deletion, fetching rows and modifying rows etc.
SQL is an ANSI (American National Standards Institute) standard but there are many different versions of the SQL language.

What is SQL?
SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in relational database.
SQL is the standard language for Relation Database System. All relational database management systems like MySQL, MS Access, Oracle, Sybase, Informix, postgres and SQL Server use SQL as standard database language.
Also, they are using different dialects, such as:
·        MS SQL Server using T-SQL,
·        Oracle using PL/SQL,
·        MS Access version of SQL is called JET SQL (native format) etc.

What is SQL?
·        Allows users to access data in relational database management systems.
·        Allows users to describe the data.
·        Allows users to define the data in 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

History:
·       1970 -- Dr. Edgar F. "Ted" Codd of IBM is known as the father of relational databases. He described a relational model for databases.
·        1974 -- Structured Query Language appeared.
·        1978 -- IBM worked to develop Codd's ideas and released a product named System/R.
·    1986 -- IBM developed the first prototype of relational database and standardized by ANSI. The first relational database was released by Relational Software and its later becoming Oracle.

SQL Process:
When you are executing an SQL command for any RDBMS, the system determines the best way to carry out your request and SQL engine figures out how to interpret the task.
There are various components included in the process. These components are Query Dispatcher, Optimization Engines, Classic Query Engine and SQL Query Engine, etc. Classic query engine handles all non-SQL queries but SQL query engine won't handle logical files.
Following is a simple diagram showing SQL Architecture:


SQL Commands:
The standard SQL commands to interact with relational databases are CREATE, SELECT, INSERT, UPDATE, DELETE and DROP. These commands can be classified into groups based on their nature:

DDL - Data Definition Language:
Command
Description
CREATE
Creates a new table, a view of a table, or other object in database
ALTER
Modifies an existing database object, such as a table.
DROP
Deletes an entire table, a view of a table or other object in the database.


DML - Data Manipulation Language:

Command
Description
SELECT
Retrieves certain records from one or more tables
INSERT
Creates a record
UPDATE
Modifies records
DELETE
Deletes records


DCL - Data Control Language: 


Command
Description
GRANT
Gives a privilege to user
REVOKE
Takes back privileges granted from user

RDBMS Concepts



What is RDBMS?

RDBMS stands for Relational Database Management System. RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.
A Relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd.

What is table?

The data in RDBMS is stored in database objects called tables. The table is a collection of related data entries and it consists of columns and rows.
Remember, a table is the most common and simplest form of data storage in a relational database. Following is the example of a CUSTOMERS table:
+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

What is field?

Every table is broken up into smaller entities called fields. The fields in the CUSTOMERS table consist of ID, NAME, AGE, ADDRESS and SALARY.
A field is a column in a table that is designed to maintain specific information about every record in the table.

What is record or row?

A record, also called a row of data, is each individual entry that exists in a table. For example there are 7 records in the above CUSTOMERS table. Following is a single row of data or record in the CUSTOMERS table:
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
+----+----------+-----+-----------+----------+
A record is a horizontal entity in a table.

What is column?

A column is a vertical entity in a table that contains all information associated with a specific field in a table.
For example, a column in the CUSTOMERS table is ADDRESS, which represents location description and would consist of the following:
+-----------+
| ADDRESS   |
+-----------+
| Ahmedabad |
| Delhi     |
| Kota      |
| Mumbai    |
| Bhopal    |
| MP        |
| Indore    |
+----+------+

What is NULL value?

A NULL value in a table is a value in a field that appears to be blank, which means a field with a NULL value is a field with no value.
It is very important to understand that a NULL value is different than a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation.

SQL Constraints:

Constraints are the rules enforced on data columns on 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 could be column level or table level. Column level constraints are applied only to one column where as table level constraints are applied to the whole table.
Following are commonly used constraints available in SQL:
  • NOT NULL Constraint: Ensures that a column cannot have NULL value.
  • DEFAULT Constraint: Provides a default value for a column when none is specified.
  • UNIQUE Constraint: Ensures that all values in a column are different.
  • PRIMARY Key: Uniquely identified each rows/records in a database table.
  • FOREIGN Key: Uniquely identified a rows/records in any another database table.
  • CHECK Constraint: The CHECK constraint ensures that all values in a column satisfy certain conditions.
  • INDEX: Use to create and retrieve data from the database very quickly.

Data Integrity:

The following categories of the 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.

Database Normalization

Database normalization is the process of efficiently organizing data in a database. There are two reasons of the normalization process:
  • Eliminating redundant data, for example, storing the same data in more than one tables.
  • Ensuring data dependencies make sense.
Both of these are worthy goals as they reduce the amount of space a database consumes and ensure that data is logically stored. Normalization consists of a series of guidelines that help guide you in creating a good database structure.
Normalization guidelines are divided into normal forms; think of form as the format or the way a database structure is laid out. The aim of normal forms is to organize the database structure so that it complies with the rules of first normal form, then second normal form, and finally third normal form.
It's your choice to take it further and go to fourth normal form, fifth normal form, and so on, but generally speaking, third normal form is enough.