Structured Query Language (SQL)

Overview

• SQL is a language used for interacting with Relational Database Management Systems (RDBMS) -You can use SQL to get the RDBMS to do things for you
  • Create, retrieve, update & delete data
  • Create & manage databases
  • Design & create database tables
  • Perform administration tasks (security, user management, import/export, etc)
• SQL implementations vary between systems
• Not all RDBMS' follow the SQL standard to a 'T'
• The concepts are the same but the implementation may vary

SQL Types

• SQL is actually a hybrid language, it's basically 4 types of languages in one

Data Query Language (DQL)

• Used to query the database for information.
• Get information that is already stored there

Data Definition Language (DDL)

• Used for defining database schemas.
• Defining what kind of schema will be there, column etc.

Data Control Language (DCL)

• Used for controlling access to the data in the database.
• User & permissions management

Data Manipulation Language (DML)

• Used for inserting, updating and deleting data from the databases

Queries

A query is a set of instructions given to the RDBMS (written is SQL) that tell the RDBMS what information you want it to retrieve for you

• TONS of data in a DB
• Often hidden in a complex schema
• Goal is to only get the data you need

For example -

SELECT employee.name, employee.age
FROM employee
WHERE employee.salary > 3000;