Introduction to SQL I

Introduction to SQL I

SQL is a domain-specific programming language designed for managing data in a relational database management system(RDBMS), and is used by a huge number of apps and organizations. For more information, please click the following link: SQL_wiki

Nowadays, people who work as data scienctist and analyst, software developer and even web designer require to use SQL as a tool for sharing and managing data in RDBMS. This post aims to record my self-learning experience and share with people who would like to learnig some basic technics in SQL. So, Let’s get started.

• About SQL
  

  SQL are compoesd of three parts

Data Definition Language (DDL)
Data Manipulation Language (DML)
Data Control Language (DCL)

Today, I will introduce the first part, DDL.

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• About Tables

Before we go through the Syntax of SQL, we whould have fundemantal understading about the Tables.

Data direction	name	also known as
horizontal	Rows	Observations
vertical	Columns	Variables

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Important Syntax in DDL

• remark: the syntax and function of SQL need to be all capitals. e.g. SELECT, FROM and DROP.
• remark2: put the semicolon (;) in the end of statement.

1. Create Statement
   • The create command is used to establish a new database, table, index, or stored procedure.

Syntax: CREATE DATABASE database-name
Example: CREATE DATABASE exerciseDB;

Syntax: CREATE TABLE table-name Example: CREATE TABLE exercise_table;

1. Delete Statement
   
   • The DROP statement destroys an existing database, table, index, or view.

Syntax: DROP DATABASE database-name
Example: DROP DATABASE exerciseDB;

Syntax: DROP TABLE table-name
Example: DROP TABLE exercise_table;

1. Alter Statement
   • The ALTER statement modifies an existing database object.

Syntax: ALTER DATABASE database-name
    MODIFY NAME = new_db;
Example: ALTER DATABASE exerciseDB
    MODIFY NAME = exerciseDB_new;

Syntax: ALTER TABLE table-name
    ADD COLUMN col_type;
Example: ALTER TABLE exercise_table
    ADD COLUMN Age;

Syntax: ALTER TABLE table-name
    ADD PRIMARY KEY col_name;
Example: ALTER TABLE exercise_table
    ADD PRIMARY KEY Age;