SQL SELECT Statement 🔍¶

Mentor's Note: If SQL is a conversation, SELECT is the question. It's how you ask the database, "Can you show me this specific information?" It is the most used command in the world of data. 💡

🌟 The Scenario: The Grocery List 🛒¶

Imagine you are in a massive supermarket.

SELECT *: "Bring me every single item in the store." (Too much! 🤯)
SELECT name, price: "Just show me the names and prices of the items." (Much better! ✅)

🎨 Visual Logic: The Extraction Process¶

graph LR
    A[(Table: Data)] -- "SELECT Filter" --> B[Result Set: View]
    subgraph "Columns"
    B1[id]
    B2[name]
    B3[salary]
    end

💻 1. The Basic Syntax¶

SELECT column1, column2, ...
FROM table_name;

A. Select All Columns (*)¶

Use the asterisk (*) to see everything.

-- ⚠️ Warning: Avoid using * in production apps! It's slow.
SELECT * FROM employees;

B. Select Specific Columns¶

Always prefer this for better performance.

SELECT first_name, email FROM employees;

💻 2. Column Aliases (AS) 🏷️¶

You can give a temporary name to a column in your result set to make it more readable.

-- Scenario: Changing technical headers to friendly ones
SELECT first_name AS "Name", salary AS "Monthly_Pay" 
FROM employees;

💻 3. Performing Calculations¶

You can perform math directly inside your SELECT statement.

-- Scenario: Calculate an annual bonus (10% of salary)
SELECT 
    first_name, 
    salary, 
    (salary * 0.10) AS bonus 
FROM employees;

💡 Pro Tip: The "Rule of SELECT"¶

"Never use SELECT * in your software code. If someone adds a new column (like a huge image blob) to the table later, your app might crash or slow down because it's fetching data it doesn't need!"