DML (Data Manipulation Language): Manage data within schema objects.

SELECT
INSERT
UPDATE
DELETE

DCL (Data Control Language): Control access to data.

GRANT
REVOKE

TCL (Transaction Control Language): Manage transactions.

COMMIT
ROLLBACK
SAVEPOINT

5. Basic SQL Commands (DDL)

Create Table:

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  FirstName VARCHAR(50),
  LastName VARCHAR(50),
  Age INT,
  City VARCHAR(50)
);

Alter Table (Add Column):

ALTER TABLE Students
ADD Email VARCHAR(100);

Alter Table (Drop Column):
```
ALTER TABLE Students
DROP COLUMN Email;
```
Drop Table:
```
DROP TABLE Students;
```
Truncate Table (Removes all rows, keeps structure):
```
TRUNCATE TABLE Students;
```

6. Basic SQL Commands (DML)

Insert Data:

INSERT INTO Students (StudentID, FirstName, LastName, Age, City)
VALUES (1, 'John', 'Doe', 20, 'New York');

Update Data:

UPDATE Students
SET Age = 21, City = 'Los Angeles'
WHERE StudentID = 1;

Delete Data:

DELETE FROM Students
WHERE StudentID = 1;

Select Data:

SELECT * FROM Students;
SELECT FirstName, City FROM Students WHERE Age > 20;

Distinct Values:
```
SELECT DISTINCT City FROM Students;
```

Order By:

SELECT * FROM Students
ORDER BY LastName ASC, FirstName DESC;

Limit/Top:

SELECT * FROM Students LIMIT 10; -- MySQL/PostgreSQL
SELECT TOP 10 * FROM Students; -- SQL Server

Aggregate Functions:
- COUNT(): Number of rows.
- SUM(): Sum of values.
- AVG(): Average value.
- MIN(): Minimum value.
- MAX(): Maximum value.
```
SELECT COUNT(StudentID), AVG(Age) FROM Students;
```

Group By & Having:

SELECT City, COUNT(StudentID) AS TotalStudents
FROM Students
GROUP BY City
HAVING COUNT(StudentID) > 1;

7. SQL Joins

Purpose: Combine rows from two or more tables based on a related column.

Types:

INNER JOIN: Returns rows when there is a match in both tables.

SELECT S.FirstName, C.CourseName
FROM Students S
INNER JOIN Enrollments E ON S.StudentID = E.StudentID
INNER JOIN Courses C ON E.CourseID = C.CourseID;

LEFT JOIN (LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table. NULL for no match.
```
SELECT S.FirstName, E.EnrollmentDate
FROM Students S
LEFT JOIN Enrollments E ON S.StudentID = E.StudentID;
```
RIGHT JOIN (RIGHT OUTER JOIN): Return