Databases

This section covers the fundamentals of databases and how to interact with them using Structured Query Language (SQL). We will focus on basic SQL queries, including SELECT, WHERE, AND, OR, and ORDER BY.

What is a Database?

A database is an organized collection of data, typically stored and accessed electronically. Databases allow for efficient storage, retrieval, and management of information.

Relational Databases

Most modern databases are relational databases. This means data is organized into tables, which have rows (records) and columns (fields). Relationships between tables are established using common fields.

Tables, Rows, and Columns

A table represents a collection of related data. Each row in a table represents a single record, and each column represents a specific attribute of that record.

Column Name	Data Type	Example
Student ID	Integer	12345
Name	Text	Alice Smith
Age	Integer	16

SQL Basics

SQL (Structured Query Language) is the standard language for interacting with relational databases. We will explore the core commands for retrieving data.

SELECT Statement

The SELECT statement is used to retrieve data from one or more tables. The basic syntax is:

SELECT column1, column2, ...
FROM table_name; 

To select all columns from a table, use SELECT *.

WHERE Clause

The WHERE clause is used to filter the data retrieved by specifying conditions. It allows you to select only rows that meet certain criteria.

Syntax: WHERE condition

Example: SELECT * FROM Students WHERE Age > 15; (selects all students older than 15)

Logical Operators: AND and OR

You can combine multiple conditions using logical operators:

• AND: Both conditions must be true.
• OR: At least one condition must be true.

Example: SELECT * FROM Students WHERE Age > 15 AND Grade = 'A'; (selects students older than 15 who have a grade of 'A')

Example: SELECT * FROM Students WHERE Age > 15 OR Grade = 'A'; (selects students older than 15 or those with a grade of 'A')

ORDER BY Clause

The ORDER BY clause is used to sort the retrieved data. You can sort by one or more columns in ascending (ASC) or descending (DESC) order.

Syntax: ORDER BY column_name [ASC | DESC]

Example: SELECT * FROM Students ORDER BY Age ASC; (sorts students by age in ascending order)

Example: SELECT * FROM Students ORDER BY Grade DESC, Name ASC; (sorts students by grade in descending order, then by name in ascending order within each grade)

Example SQL Queries

Query	Description
SELECT Name, Age FROM Students;	Retrieves the name and age of all students.
SELECT * FROM Students WHERE Age >= 16;	Retrieves all information for students aged 16 or older.
SELECT * FROM Students WHERE Age > 16 AND Grade = 'B';	Retrieves all information for students older than 16 with a grade of 'B'.
SELECT * FROM Students WHERE Age > 16 OR Grade = 'B';	Retrieves all information for students older than 16 or those with a grade of 'B'.
SELECT Name, Age FROM Students ORDER BY Age DESC;	Retrieves the name and age of all students, sorted by age in descending order.
SELECT * FROM Students ORDER BY Grade DESC, Name ASC;	Retrieves all information for students, sorted by grade in descending order, and then by name in ascending order within each grade.

Further Learning

This is a basic introduction to SQL queries. You can explore more advanced concepts such as joins, aggregate functions, and subqueries for a deeper understanding of database interaction.