Data Science & Machine Learnin… on Telegram: “order_id | customer_id | product | price 1

order_id | customer_id | product | price
1        | 101         | Laptop  | 60000
2        | 102         | Phone   | 30000
3        | 101         | Mouse   | 1000

To retrieve order details:

SELECT
    order_id,
    customer_id,
    product,
    price
FROM orders;

To calculate price after adding a hypothetical 10% increase:

SELECT
    product,
    price,
    price * 1.10 AS increased_price
FROM orders;

🔹 21. SELECT in Real-World Data Science

SQL is often the first step in a Data Science workflow.

For example:

Business Question

"Give me all customer transactions from the sales database."

You might start with:

SELECT
    customer_id,
    order_date,
    product_id,
    amount
FROM transactions;

Then later:

WHERE → Filter data

GROUP BY → Aggregate data

JOIN → Combine tables

ORDER BY → Sort results

Window Functions → Advanced analysis

Eventually, you may load the SQL result into Pandas:

import pandas as pd
df = pd.read_sql(query, connection)

So SQL and Python often work together.

🔹 22. Common Beginner Mistakes

❌ Mistake 1: Forgetting the FROM clause

Incorrect:

SELECT name;

when you intend to retrieve a column from a table.

Correct:

SELECT name
FROM customers;

❌ Mistake 2: Using commas incorrectly

Correct:

SELECT name, city, age
FROM customers;

❌ Mistake 3: Confusing column names and values

A column:

city

is different from a text value:

'Pune'

We'll explore this more when we learn WHERE.

❌ Mistake 4: Using SELECT * everywhere

SELECT * is useful while learning and exploring, but for production queries, selecting only the required columns can be more efficient and clearer.

🔹 23. Interview Questions

💡 What does SELECT do?

SELECT specifies the columns or expressions that should appear in the query result.

💡 What does SELECT * mean?

It selects all columns from the specified table.

💡 What is an alias?

An alias gives a temporary name to a column or expression in the query result.

💡 What does DISTINCT do?

It removes duplicate rows from the selected result.

🎯 Practice Questions

Q1. Write a query to select all columns from a table called employees.

Q2. Write a query to select only employee_id and salary from employees.

Q3. Write a query to display salary as monthly_salary.

Q4. Write a query to calculate price * quantity as total_amount from an orders table.

Q5. Write a query to return unique values from the department column of an employees table.

🎯 Key Takeaways

✅ SQL is used to communicate with relational databases.

✅ SELECT specifies what you want to retrieve.

✅ FROM specifies the table.

✅ * means all columns.

✅ You can select one or multiple columns.

✅ AS creates a temporary alias.

✅ SQL can perform calculations.

✅ DISTINCT returns unique results.

✅ SQL is one of the most important tools for extracting data before analysis and Machine Learning.

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