What is SQL?

 

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🔷 What is SQL?

SQL (Structured Query Language) is a standard language used to store, manage, and retrieve data in a relational database.

Popular SQL databases include:

• MySQL
• PostgreSQL
• SQL Server
• Oracle
• SQLite

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🔷 What SQL is Used For

SQL helps you:

1. Create databases and tables
2. Insert data
3. Update or delete data
4. Search/filter data
5. Join multiple tables
6. Control access to data

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🔷 Most Common SQL Commands

1️⃣ CREATE – Create a table

CREATE TABLE Students (
    ID INT,
    Name VARCHAR(50),
    Age INT
);

2️⃣ INSERT – Add data to table

INSERT INTO Students (ID, Name, Age)
VALUES (1, 'Amit', 20);

3️⃣ SELECT – Retrieve data

SELECT * FROM Students;

4️⃣ WHERE – Filter results

SELECT * FROM Students
WHERE Age > 18;

5️⃣ UPDATE – Modify data

UPDATE Students
SET Age = 21
WHERE ID = 1;

6️⃣ DELETE – Remove data

DELETE FROM Students
WHERE ID = 1;

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🔷 Basic SQL Concepts

Concept	Meaning
Table	Collection of rows and columns
Row	A single record
Column	Field/attribute
Primary Key	Unique identifier for each row
Foreign Key	Links tables together
Join	Combine data from multiple tables

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🔷 Example: JOIN

SELECT Orders.OrderID, Customers.Name
FROM Orders
JOIN Customers
ON Orders.CustomerID = Customers.ID;

Great! Here is More SQL content, including joins, functions, constraints, and practice questions—perfect for beginners to intermediate learners.

🔷 1. SQL JOINS (Very Important)
Joins are used to combine data from multiple tables.
✅ INNER JOIN
Returns matching rows from both tables.
SELECT s.Name, c.CourseName
FROM Students s
INNER JOIN Courses c
ON s.CourseID = c.CourseID;

✅ LEFT JOIN
Returns all rows from left table + matching rows from right.
SELECT s.Name, c.CourseName
FROM Students s
LEFT JOIN Courses c
ON s.CourseID = c.CourseID;

✅ RIGHT JOIN
Returns all rows from right table + matching rows from left.
SELECT s.Name, c.CourseName
FROM Students s
RIGHT JOIN Courses c
ON s.CourseID = c.CourseID;

✅ FULL OUTER JOIN
Returns all rows when there is a match in either table.
SELECT s.Name, c.CourseName
FROM Students s
FULL OUTER JOIN Courses c
ON s.CourseID = c.CourseID;


🔷 2. SQL Constraints


Constraints ensure rules in a table.



Constraint
Description




PRIMARY KEY
Unique + Not Null


FOREIGN KEY
Links tables


UNIQUE
No duplicate values


NOT NULL
Cannot be empty


CHECK
Ensures a condition


DEFAULT
Assigns default value



Example:
CREATE TABLE Employees (
    EmpID INT PRIMARY KEY,
    Name VARCHAR(50) NOT NULL,
    Age INT CHECK (Age > 18),
    City VARCHAR(50) DEFAULT 'Unknown'
);


🔷 3. SQL Aggregate Functions
Used to perform calculations on data.



Function
Meaning




COUNT()
Counts rows


SUM()
Adds values


AVG()
Average


MIN()
Minimum


MAX()
Maximum



Example:
SELECT COUNT(*), AVG(Age)
FROM Students;


🔷 4. SQL GROUP BY & HAVING
GROUP BY
Used to group rows that have the same values.
SELECT City, COUNT(*)
FROM Students
GROUP BY City;

HAVING
Used for filtering groups (not rows).
SELECT City, COUNT(*)
FROM Students
GROUP BY City
HAVING COUNT(*) > 2;


🔷 5. SQL ORDER BY
SELECT * FROM Students
ORDER BY Age DESC;


🔷 6. SQL Subqueries
Subquery inside another query.
SELECT Name
FROM Students
WHERE Age > (SELECT AVG(Age) FROM Students);


🔷 7. SQL Normalization (Basics)
Normalization reduces redundancy.



Normal Form
Meaning




1NF
No repeating groups, atomic values


2NF
No partial dependency


3NF
No transitive dependency




🔷 8. SQL Practice Questions (Very Useful)
Try these yourself:
Q1. Display employee name and salary who earns more than average salary.
Q2. List customers who have never placed an order.
Q3. Find the second highest salary.
Q4. Count number of students in each city.
Q5. Write a query to delete duplicate records.