🗄️ How to Solve SQL Problems

If you are a beginner, don't try to write the entire SQL query immediately. The easiest approach is to break the problem into small steps.

📌 Step 1: Understand What the Question Is Asking

Read the question carefully and identify the final output.

Example:

Find the total sales for each customer.

Ask yourself:

👉 What do I need to display?

Answer:

Customer

Total Sales

📌 Step 2: Identify the Table

Find which table contains the required information.

Suppose you have:

sales

customer_id

product

quantity

price

You need the sales table.

📌 Step 3: Identify the Required Columns

For:

Find total sales for each customer.

You need:

customer_id

quantity

price

Because: Sales = quantity × price

📌 Step 4: Decide Whether You Need Filtering

Ask:

Do I need only certain rows?

For example:

Find total sales for customers who purchased in 2026.

Now you need a WHERE condition.

WHERE order_date >= '2026-01-01'

📌 Step 5: Decide Whether You Need GROUP BY

Look for words such as: Each customer, Each department, Per product, By region, By month

These usually indicate GROUP BY.

For example:

Find total sales for each customer.

GROUP BY customer_id

📌 Step 6: Identify the Required Aggregate Function

Look for words like:

Total → SUM()

Average → AVG()

Count → COUNT()

Maximum → MAX()

Minimum → MIN()

For total sales:

SUM(quantity _ price)

📌 Step 7: Build the Query Step by Step

Instead of writing everything at once:

1.

SELECT customer_id FROM sales;

2.

Add the calculation:

SELECT customer_id, SUM(quantity _ price) AS total_sales FROM sales;

3.

Add grouping:

SELECT

customer_id,

SUM(quantity ** price) AS total_sales

FROM sales

GROUP BY customer_id;

Now the query is complete.

📌 Step 8: Check Whether You Need HAVING

Suppose the question changes to:

Find customers whose total sales are greater than ₹50,000.

You cannot use WHERE on SUM(). Use HAVING:

SELECT

customer_id,

SUM(quantity ** price) AS total_sales

FROM sales

GROUP BY customer_id

HAVING SUM(quantity ** price) > 50000;

📌 Step 9: Check Whether You Need a JOIN

Suppose the question says:

Find the names of customers and their total sales.

You have:

customers: customer_id, customer_name

sales: customer_id, quantity, price

Now you need a JOIN.

SELECT

c.customer_name,

SUM(s.quantity ** s.price) AS total_sales

FROM customers c

JOIN sales s

ON c.customer_id = s.customer_id

GROUP BY c.customer_name;

📌 Step 10: Validate Your Answer

Before considering the problem solved, check:

✓ Did I use the correct table?

✓ Did I select the correct columns?

✓ Is my JOIN correct?

✓ Did I handle NULL values?

✓ Did I accidentally create duplicates?

✓ Did I use WHERE or HAVING correctly?

✓ Does the output actually answer the question?

🧠 Use This SQL Problem-Solving Framework

Whenever you get a SQL question, think:

1. What is being asked?

2. Which table(s) do I need?

3. Which columns do I need?

4. Do I need filtering?

5. Do I need a JOIN?

6. Do I need aggregation?

7. Do I need GROUP BY?

8. Do I need HAVING?

9.

Do I need a window function?

10. Validate the result

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