`SELECT` – Reading Data

🟢 Beginner

📖 Definition

The SELECT statement is a Data Query Language (DQL) command used to query and retrieve data rows from one or more database tables.

🇮🇳 Hindi

Database tables se data read aur retrieve karne ke liye SELECT query ka use hota hai. Aap saare columns (SELECT *) ya specific columns (SELECT col1, col2) choose kar sakte hain. Duplicate rows hatane ke liye DISTINCT aur column name change karke dikhane ke liye Column Alias (AS) ka use hota hai.

🚩 Marathi

Database table madhun data vachnyasathi aani baghnyasathi SELECT query cha wapar kela jato. Distinct (unique) values sathi DISTINCT aani navin nav dhenyasathi AS (Alias) vaparatat.

📝 Key Features & Syntax Variations

1. Selecting All Columns (`SELECT *`)

Retrieves every column defined in the table:

SELECT * FROM employees;

2. Selecting Specific Columns (Best Practice for Performance)

SELECT first_name, last_name, salary FROM employees;

3. Column Aliases (`AS`)

Renames output column headings in the query result table:

SELECT first_name AS Name, salary AS MonthlySalary FROM employees;

4. Unique Values (`DISTINCT`)

Filters out duplicate row values from result set:

SELECT DISTINCT department FROM employees;

5. Calculated / Derived Columns

Performs calculations directly inside the query:

SELECT 
    first_name, 
    salary, 
    (salary * 12) AS annual_salary,
    (salary * 0.10) AS bonus_amount
FROM employees;

💡 Practical Example: Advanced SELECT Query

-- Sample Employees Table
CREATE TABLE employees (
    emp_id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    department VARCHAR(50),
    salary DECIMAL(10, 2)
);

INSERT INTO employees VALUES 
(1, 'Rahul', 'Dravid', 'Sports', 85000.00),
(2, 'Priya', 'Sharma', 'HR', 60000.00),
(3, 'Amit', 'Kumar', 'Sports', 85000.00),
(4, 'Sneha', 'Patel', 'Engineering', 95000.00),
(5, 'Kiran', 'Rao', 'HR', 60000.00);

-- Query 1: Selecting specific columns with Alias & Calculations
SELECT 
    CONCAT(first_name, ' ', last_name) AS full_name,
    department,
    salary,
    (salary * 1.10) AS salary_after_hike
FROM employees;

-- Query 2: Getting unique departments
SELECT DISTINCT department FROM employees;

👀 Output

Query 1 Output:
+--------------+-------------+----------+-------------------+

| full_name    | department  | salary   | salary_after_hike |
+--------------+-------------+----------+-------------------+

| Rahul Dravid | Sports      | 85000.00 |          93500.00 |
| Priya Sharma | HR          | 60000.00 |          66000.00 |
| Amit Kumar   | Sports      | 85000.00 |          93500.00 |
| Sneha Patel  | Engineering | 95000.00 |         104500.00 |
| Kiran Rao    | HR          | 60000.00 |          66000.00 |
+--------------+-------------+----------+-------------------+

Query 2 Output (DISTINCT):
+-------------+

| department  |
+-------------+

| Sports      |
| HR          |
| Engineering |
+-------------+

⚠️ Common Mistakes

Overusing SELECT * in production application code. Selecting unnecessary large text or blob columns degrades database and network bandwidth performance!
Forgetting that DISTINCT applies across all selected columns combined, not just the first column.

🌍 Real-World Usage

Every web app dashboard, search results page, report generator, and data visualization analytics tool runs SELECT queries to display information to users.

🧪 Try It Yourself

Write a query to select first_name and salary from employees with an alias Monthly_Pay.
Write a query to find all unique job departments in your organization.

🎯 Mini Challenge

Write a SELECT query that calculates an employee’s daily pay assuming 22 working days per month (salary / 22) aliased as daily_rate.

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SELECT – Reading Data