SQL Overview

Structured Query Language (SQL) is a standardized programming language that is used to manage and manipulate data held in a relational database. SQL is used to access and manipulate data. It's a declarative programming language, which means you tell the database what you want, but not how to get it.

Core Concepts of SQL

SQL is built around a few fundamental concepts:

Tables: Data is organized into tables, which consist of rows and columns.
Columns (Attributes): Represent individual pieces of data within a table. Each column has a specific data type (e.g., INTEGER, VARCHAR, DATE).
Rows (Records/Tuples): Represent a single entry or item in a table.
Primary Key: A column or a set of columns that uniquely identifies each row in a table.
Foreign Key: A column or a set of columns in one table that refers to the primary key in another table, establishing a link between them.

Key SQL Commands (DDL & DML)

Data Definition Language (DDL)

DDL statements are used to define, modify, and delete database objects.

CREATE: Used to create database objects like tables, indexes, and views.

CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Email VARCHAR(100)
);

ALTER: Used to modify the structure of an existing database object.
```
ALTER TABLE Customers
ADD PhoneNumber VARCHAR(20);
```
DROP: Used to delete a database object.
```
DROP TABLE Customers;
```

Data Manipulation Language (DML)

DML statements are used to manage data within database objects.

SELECT: Used to retrieve data from one or more tables. This is the most frequently used SQL command.
```
SELECT FirstName, LastName
FROM Customers
WHERE CustomerID = 10;
```

INSERT: Used to add new records (rows) into a table.

INSERT INTO Customers (CustomerID, FirstName, LastName, Email)
VALUES (101, 'John', 'Doe', 'john.doe@example.com');

UPDATE: Used to modify existing records in a table.

UPDATE Customers
SET Email = 'jane.smith@newdomain.com'
WHERE CustomerID = 101;

DELETE: Used to remove records from a table.

DELETE FROM Customers
WHERE CustomerID = 101;

A Simple SQL Query Example

Let's say we have a Products table:

Products (ProductID INT PRIMARY KEY, ProductName VARCHAR(100), Price DECIMAL(10, 2), Stock INT);

To find the names and prices of products that cost more than $50:

SELECT ProductName, Price
FROM Products
WHERE Price > 50.00;

SQL Clauses and Operators

SQL uses various clauses and operators to refine queries:

WHERE: Filters records based on specified conditions.
ORDER BY: Sorts the result set by one or more columns.
GROUP BY: Groups rows that have the same values in specified columns.
HAVING: Filters groups based on specified conditions (used with GROUP BY).
JOIN: Combines rows from two or more tables based on a related column. (See SQL Joins for more details).

Common operators include comparison operators (=, >, <, >=, <=, <> or !=), logical operators (AND, OR, NOT), and pattern matching (LIKE).

Why Learn SQL?

SQL is a fundamental skill for anyone working with data. It's used extensively in:

Web Development (backend)
Data Analysis
Business Intelligence
Database Administration
Software Engineering

Most modern database systems, such as MySQL, PostgreSQL, SQL Server, Oracle, and SQLite, use SQL as their standard query language.

This overview provides a foundational understanding of SQL. For more advanced topics and specific database implementations, please refer to the subsequent sections.