SQL Introduction

SQL (Structured Query Language) is a standardized programming language used for managing and manipulating relational databases. It provides a way to interact with databases through commands that allow users to query, update, insert, and delete data, as well as create and modify database structures.

 

Key Features of SQL

Database Interaction

• SQL is primarily used for interacting with relational databases such as MySQL, PostgreSQL, Microsoft SQL Server, and SQLite.
• Benefit: Provides a way to work with structured data and easily manage large datasets.

Data Querying

• SQL allows users to retrieve specific data from a database using the SELECT statement.
• Benefit: Enables complex querying and data extraction for analysis, reporting, and decision-making.

Data Manipulation

• SQL includes commands like INSERT, UPDATE, and DELETE to modify the data within the database.
• Benefit: Makes it easy to add, update, or remove data records in real time.

Database Structure Management

• SQL provides commands like CREATE, ALTER, and DROP to define and modify the structure of database tables and other objects.
• Benefit: Helps organize and manage the database schema, ensuring efficient data storage.

Data Integrity and Constraints

• SQL supports the definition of constraints like PRIMARY KEY, FOREIGN KEY, NOT NULL, and UNIQUE to ensure the accuracy and reliability of the data.
• Benefit: Ensures that data adheres to specific rules and prevents inconsistent data from being stored.

 

Basic SQL Commands

SELECT: Used to query data from a table.

SELECT * FROM Customers;

INSERT INTO: Used to insert new records into a table.

INSERT INTO Customers (Name, Age) VALUES ('John Doe', 30);

UPDATE: Used to modify existing data in a table.

UPDATE Customers SET Age = 31 WHERE Name = 'John Doe';

DELETE: Used to delete records from a table.

DELETE FROM Customers WHERE Name = 'John Doe';

CREATE TABLE: Used to create a new table in the database.

CREATE TABLE Customers (
    ID INT PRIMARY KEY,
    Name VARCHAR(50),
    Age INT
);

 

Types of SQL Queries

Data Retrieval

• SELECT: Retrieves data from one or more tables.
• Example:

SELECT Name, Age FROM Customers WHERE Age > 25;

Data Insertion

• INSERT INTO: Adds new rows to a table.
• Example:

INSERT INTO Customers (Name, Age) VALUES ('Jane Smith', 28);

Data Update

• UPDATE: Modifies existing data in a table.
• Example:

UPDATE Customers SET Age = 29 WHERE Name = 'Jane Smith';

Data Deletion

• DELETE: Removes data from a table.
• Example:

DELETE FROM Customers WHERE Name = 'Jane Smith';

 

SQL Clauses and Operators

WHERE: Filters records based on specified conditions.

• Example:

SELECT * FROM Customers WHERE Age > 30;

ORDER BY: Sorts the results in ascending or descending order.

• Example:

SELECT * FROM Customers ORDER BY Age DESC;

AND/OR: Combines multiple conditions in the WHERE clause.

• Example:

SELECT * FROM Customers WHERE Age > 25 AND Name = 'John Doe';

LIKE: Searches for a specified pattern in a column.

• Example:

SELECT * FROM Customers WHERE Name LIKE 'J%';

SQL Functions

COUNT(): Returns the number of rows in a result set.

SELECT COUNT(*) FROM Customers;

AVG(): Calculates the average value of a numeric column.

SELECT AVG(Age) FROM Customers;

MAX(): Returns the maximum value of a column.

SELECT MAX(Age) FROM Customers;

MIN(): Returns the minimum value of a column.

SELECT MIN(Age) FROM Customers;

SUM(): Returns the sum of a numeric column.

SELECT SUM(Age) FROM Customers;

 

Database Relationships

1. One-to-Many Relationship: One record in a table is related to many records in another table.
2. Many-to-Many Relationship: Multiple records in one table can be related to multiple records in another table. This is typically achieved using a join table.

 

Summary

SQL is a powerful language used to manage and manipulate relational databases. It allows developers to interact with data by querying, updating, inserting, and deleting records. SQL's flexibility in querying and managing data, combined with its robust support for constraints and data integrity, makes it an essential skill for working with databases. Understanding key SQL commands and functions can help you build, manage, and optimize data-driven applications.