What is SQL?

SQL (Structured Query Language) is the language used to communicate with a relational database.

Think of it like this:

Java  ---> JVM
Browser ---> Web Server
SQL ---> Database

Whenever you want to:

You use SQL.

Example

Suppose we have this table.

Employees

Employee_ID Name Age Department Salary
101 Rahul 25 IT 60000
102 Priya 28 HR 55000
103 Amit 30 IT 70000
104 Neha 24 Sales 50000
105 Rohan 29 Finance 65000

Now suppose you ask:

Show all employees.

SQL:

SELECT * FROM Employees;

SQL is simply a language for asking questions to the database.

Categories of SQL Commands

Interviewers often ask:

How many types of SQL commands are there?

There are five categories.

Category Purpose
DDL Define database structure
DML Manipulate data
DQL Retrieve data
DCL Control permissions
TCL Manage transactions

1. DDL (Data Definition Language)

Used to create or modify the database structure.

Commands:

Create a table:

CREATE TABLE Employees (
    Employee_ID INT,
    Name VARCHAR(50),
    Age INT
);

Add a new column:

ALTER TABLE Employees
ADD Salary INT;

Delete the table completely:

DROP TABLE Employees;

Remove all rows but keep the table:

TRUNCATE TABLE Employees;

Rename the table:

RENAME TABLE Employees TO Employee;

2. DML (Data Manipulation Language)

Used to work with the data inside tables.

Commands:

Insert:

INSERT INTO Employees
VALUES (101,'Rahul',25,'IT',60000);

Update:

UPDATE Employees
SET Salary = 70000
WHERE Employee_ID = 101;

Delete:

DELETE FROM Employees
WHERE Employee_ID = 101;

Notice: the table still exists. Only rows are affected.

3. DQL (Data Query Language)

Used to retrieve data.

Command:

Example:

SELECT * FROM Employees;

Only reads data. It doesn't modify anything.

4. DCL (Data Control Language)

Used to give or remove permissions.

Commands:

Allow a user to read a table:

GRANT SELECT
ON Employees
TO Rahul;

Remove permission:

REVOKE SELECT
ON Employees
FROM Rahul;

Mostly used by Database Administrators.

5. TCL (Transaction Control Language)

Used to manage transactions.

Commands:

Example:

UPDATE Employees
SET Salary = Salary + 5000;

If everything looks good:

COMMIT;

If something goes wrong:

ROLLBACK;

Transactions are covered in detail later.

SELECT Statement

The most used SQL statement.

General syntax:

SELECT column1, column2
FROM table_name;

Example:

SELECT Name, Salary
FROM Employees;

SELECT *

Returns every column.

SELECT *
FROM Employees;

Should we use SELECT *?

In interviews: yes, it is fine for quick examples.

In production code: usually no.

Why?

Suppose a table has 50 columns, but your application only needs Name and Salary.

Instead of:

SELECT *
FROM Employees;

Write:

SELECT Name, Salary
FROM Employees;

Benefits:

SQL Execution Order

Many people think SQL executes left to right. It doesn't.

For this query:

SELECT Name
FROM Employees
WHERE Department = 'IT';

The logical execution order is:

1. FROM Employees
        |
        v
2. WHERE Department = 'IT'
        |
        v
3. SELECT Name

This explains why the database first identifies the source table, then filters rows, and only then returns the requested columns.

SQL is Case Insensitive

These are equivalent:

SELECT * FROM Employees;

select * from employees;

However, the convention is:

Example:

SELECT employee_id, employee_name
FROM employees;

Summary Table

Command Purpose
CREATE Create a table
ALTER Modify a table
DROP Delete a table
TRUNCATE Remove all rows, keep the table
INSERT Add new rows
UPDATE Modify existing rows
DELETE Remove rows
SELECT Retrieve data
GRANT Give permissions
REVOKE Remove permissions
COMMIT Save transaction
ROLLBACK Undo transaction

Interview Questions

1. What is SQL?

SQL is the standard language used to create, retrieve, update, and manage data in relational databases.

2. What are the five categories of SQL commands?

3. What is the difference between DELETE, TRUNCATE, and DROP?

DELETE TRUNCATE DROP
Removes selected rows Removes all rows Deletes the entire table
Can use WHERE No WHERE Removes table structure too
Table remains Table remains Table no longer exists
Generally logged row by row Typically minimally logged, DBMS-dependent Removes metadata and data

4. Why should we avoid SELECT * in production?

Because it retrieves unnecessary columns, increasing network transfer, memory usage, and potentially reducing query performance.

Next Topic

The next step is SQL Filtering & Sorting, where you'll use:

These are the commands you'll use in almost every SQL query.