Data Science & Machine Learning

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Used when you want to perform calculations for each department. Simple rule: DISTINCT → Give me unique values GROUP BY → Give me groups so I can calculate something 🔹 Real-World Data Science Example Imagine an e-commerce dataset. You want to know the total revenue generated by each product category.

SELECT category,
       SUM(quantity * price) AS total_revenue
FROM sales
GROUP BY category
ORDER BY total_revenue DESC;

This gives you a category-level summary that can be used for: • Revenue analysis • Product performance • Business reporting • Feature creation • Exploratory data analysis 🔹 Common Mistake Consider:

SELECT department, employee_name, AVG(salary)
FROM employees
GROUP BY department;

This can cause an error in many SQL databases because employee_name is neither: • included in GROUP BY, nor • used inside an aggregate function. A valid query would be:

SELECT department, AVG(salary)
FROM employees
GROUP BY department;

🔹 Important Rule When using GROUP BY, selected columns generally need to be either: 1. Included in the GROUP BY, or 2. Wrapped inside an aggregate function. Example:

SELECT department, COUNT(*)
FROM employees
GROUP BY department;

department → grouped column COUNT(*) → aggregate 🔹 Interview Questions 1. What is GROUP BY? GROUP BY groups rows with the same values so aggregate calculations can be performed for each group. 2. Which aggregate functions are commonly used with GROUP BY? COUNT(), SUM(), AVG(), MIN(), and MAX(). 3. What is the difference between WHERE and GROUP BY? WHERE filters individual rows before grouping. GROUP BY creates groups from the filtered rows. 4. Can GROUP BY use multiple columns? Yes. GROUP BY department, job_title; 5. Can GROUP BY be used without aggregate functions? Yes, although DISTINCT is often clearer when the goal is simply to return unique combinations. 🎯 Practice Questions 1. Find the number of employees in each department. 2. Find the average salary for each department. 3. Find the total sales for each region. 4. Find the highest salary in each department. 5. Find the total revenue for each product category and sort categories from highest to lowest revenue. 🎯 Key Takeaways • GROUP BY creates groups of similar rows. • It is commonly used with aggregate functions. • COUNT() counts. • SUM() totals. • AVG() calculates averages. • MIN() finds the minimum. • MAX() finds the maximum. • WHERE filters rows before grouping. • ORDER BY can sort grouped results. • GROUP BY is one of the most important SQL concepts for Data Science and Analytics. 📍Double Tap ❤️ For More ----- 1.34 ₽ · /balance_help

🚀 Data Science Roadmap 2026 📍 Phase 3: SQL for Data Science 📖 Topic 6 — GROUP BY GROUP BY is used to group rows that have the same value in one or more columns. It is mainly used when you want to perform calculations for each group. For example: • Total sales by region • Average salary by department • Number of customers by city • Total orders by product • Maximum transaction by category 🔹 Basic Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
GROUP BY column_name;

Common aggregate functions: • COUNT() → Count rows • SUM() → Calculate total • AVG() → Calculate average • MIN() → Find minimum • MAX() → Find maximum 🔹 Simple Example Suppose we have:

department   salary
IT           60000
IT           80000
HR           50000
HR           70000
Finance      90000

Query:

SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department;

Result:

department   avg_salary
IT           70000
HR           60000
Finance      90000

Instead of calculating one overall average, SQL calculates an average for each department. 🔹 GROUP BY with COUNT() To count employees in each department:

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;

Result:

department   employee_count
IT           2
HR           2
Finance      1

🔹 GROUP BY with SUM() Suppose you have sales data:

region   sales
North    10000
North    15000
South    20000
South    12000

Query:

SELECT region, SUM(sales) AS total_sales
FROM sales
GROUP BY region;

Result:

North   25000
South   32000

This is extremely common in business analytics. 🔹 GROUP BY with MAX() Find the highest salary in each department:

SELECT department, MAX(salary) AS highest_salary
FROM employees
GROUP BY department;

🔹 GROUP BY with MIN() Find the lowest salary in each department:

SELECT department, MIN(salary) AS lowest_salary
FROM employees
GROUP BY department;

🔹 GROUP BY with Multiple Columns You can group by multiple columns.

SELECT department, job_title, COUNT(*) AS employee_count
FROM employees
GROUP BY department, job_title;

Here SQL creates groups based on: department + job_title For example:

IT        Analyst       5
IT        Developer     8
HR        Analyst       3
Finance   Manager       4

🔹 GROUP BY with WHERE WHERE filters rows before grouping.

SELECT department, AVG(salary) AS avg_salary
FROM employees
WHERE salary > 50000
GROUP BY department;

🔹 GROUP BY with ORDER BY You can sort grouped results. For example, find departments with the highest total salary first:

SELECT department, SUM(salary) AS total_salary
FROM employees
GROUP BY department
ORDER BY total_salary DESC;

This is very useful for ranking groups. 🔹 GROUP BY vs DISTINCT These two can sometimes look similar. DISTINCT

SELECT DISTINCT department
FROM employees;

Used to find unique departments. GROUP BY

SELECT department, COUNT(*)
FROM employees
GROUP BY department;

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ORDER BY salary DESC
LIMIT 5;

Mistake 3 — Forgetting that LIMIT applies after sorting For a top-N analysis, think: ORDER BY → LIMIT not simply: LIMIT → assume top records 🔹 Interview Questions 1. What is LIMIT? LIMIT restricts the number of rows returned by a query. 2. How do you find the top 10 salaries?

SELECT *
FROM employees
ORDER BY salary DESC
LIMIT 10;

3. Can LIMIT be used with WHERE? Yes.

SELECT *
FROM employees
WHERE department = 'Finance'
LIMIT 5;

4. Why is ORDER BY important when using LIMIT? Because without sorting, the returned rows are not necessarily the highest, lowest, newest, or oldest records you want. 5. What is OFFSET used for? OFFSET skips a specified number of rows before returning the requested rows, commonly for pagination. 🎯 Double Tap ❤️ For More ----- 1.31 ₽ · /balance_help

🚀 Data Science Roadmap 2026 📍 Phase 3: SQL for Data Science 📖 Topic 4 — LIMIT LIMIT is used to control how many rows a SQL query returns. It is especially useful when you want to see only the first few records, such as: • Top 5 highest salaries • Top 10 products by sales • First 20 customers • Latest 10 transactions 🔹 Basic Syntax

SELECT *
FROM employees
LIMIT 5;

This returns only 5 rows from the result. 🔹 LIMIT with ORDER BY This is where LIMIT becomes especially powerful. Suppose you want the 5 highest-paid employees:

SELECT employee_name, salary
FROM employees
ORDER BY salary DESC
LIMIT 5;

The logical idea is: Filter → Sort → Take required number of rows For example:

Salary
120000
110000
95000
90000
85000

Only the top 5 rows are returned. 🔹 LIMIT Without ORDER BY You can write:

SELECT *
FROM employees
LIMIT 5;

But without ORDER BY, you should not assume these are the "first" or "top" 5 meaningful records. The database does not guarantee a particular order unless you explicitly specify one. So for analysis, this is usually better:

SELECT *
FROM employees
ORDER BY salary DESC
LIMIT 5;

🔹 LIMIT with WHERE You can combine WHERE, ORDER BY, and LIMIT. Example:

SELECT employee_name, salary
FROM employees
WHERE department = 'IT'
ORDER BY salary DESC
LIMIT 3;

This finds the 3 highest-paid employees in the IT department. 🔹 LIMIT for Top-N Analysis Top-N queries are extremely common in data analysis. Top 10 products by sales

SELECT product_name, sales
FROM products
ORDER BY sales DESC
LIMIT 10;

5 most recent transactions

SELECT transaction_id, transaction_date, amount
FROM transactions
ORDER BY transaction_date DESC
LIMIT 5;

3 cheapest products

SELECT product_name, price
FROM products
ORDER BY price ASC
LIMIT 3;

🔹 LIMIT with OFFSET OFFSET allows you to skip a certain number of rows before returning results. Example:

SELECT *
FROM employees
LIMIT 5 OFFSET 10;

This means: Skip 10 rows ↓ Return the next 5 rows This is commonly used for pagination. For example: • Page 1 → LIMIT 10 OFFSET 0 • Page 2 → LIMIT 10 OFFSET 10 • Page 3 → LIMIT 10 OFFSET 20 Syntax can vary between SQL databases, so always check the specific database you are using. 🔹 LIMIT vs TOP vs FETCH Different SQL databases may use different syntax. MySQL/PostgreSQL/SQLite commonly use: LIMIT 5; SQL Server commonly uses:

SELECT TOP 5 *
FROM employees;

Standard SQL also supports: FETCH FIRST 5 ROWS ONLY; The concept is the same: restrict the number of returned rows. 🔹 Real-World Data Science Example Suppose an analyst wants to investigate the 10 largest customer transactions.

SELECT customer_id,
       transaction_id,
       amount
FROM transactions
ORDER BY amount DESC
LIMIT 10;

This can help identify: • High-value transactions • Potential anomalies • Important customers • Transactions requiring further investigation 🔹 Common Mistakes Mistake 1 — Using LIMIT without sorting

SELECT *
FROM products
LIMIT 10;

This does not necessarily mean "10 best products." For that:

ORDER BY sales DESC
LIMIT 10;

Mistake 2 — Using ASC when you need the highest values Wrong:

ORDER BY salary ASC
LIMIT 5;

This returns the lowest salaries. For the highest salaries:

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🔹 Real-World Data Science Example Suppose you are analyzing sales data. You want to identify the highest-value transactions.

SELECT customer_id, product_id, quantity, price,
       quantity * price AS transaction_value
FROM sales
ORDER BY transaction_value DESC;

This can help with: • Finding high-value transactions • Identifying important customers • Investigating unusually large purchases • Preparing data for further analysis 🔹 Common Mistakes Mistake 1 — Forgetting DESC If you want the highest salary first: ORDER BY salary DESC; Not ORDER BY salary; Mistake 2 — Wrong column Make sure the column used for sorting actually represents what you want to analyze. Mistake 3 — Incorrect multiple-column ordering ORDER BY department, salary DESC; does NOT mean both columns are descending. It means: department → ASC, salary → DESC If you want both descending: ORDER BY department DESC, salary DESC; Mistake 4 — Confusing WHERE and ORDER BY WHERE filters rows. ORDER BY sorts rows. They perform different jobs. 🔹 Interview Questions 1. What is ORDER BY? ORDER BY sorts query results based on one or more columns. 2. What is the default sorting order? Ascending order (ASC) is generally the default. 3. How do you find the highest salary?

SELECT * FROM employees ORDER BY salary DESC;

4. Can you sort using multiple columns? Yes. ORDER BY department ASC, salary DESC; 5. What is the difference between ASC and DESC? ASC sorts from low to high or A–Z. DESC sorts from high to low or Z–A. 🎯 Practice Questions 1. Write a query to display all products from the cheapest to the most expensive. 2. Write a query to display employees from the highest salary to the lowest salary. 3. Write a query to display customers alphabetically by name. 4. Write a query to sort sales by date, showing the newest sales first. 5. Write a query to sort employees by department alphabetically and salary from highest to lowest within each department. 🎯 Key Takeaways • ORDER BY sorts query results • ASC means ascending • DESC means descending • Ascending is generally the default • You can sort numbers, text and dates • You can sort using multiple columns • WHERE filters data; ORDER BY sorts data • ORDER BY is especially useful when analyzing rankings and top/bottom records 👉 Double Tap ❤️ For More ----- 1.16 ₽ · /balance_help