🗄️ How to Approach SQL Problems

When you see a SQL question, don't try to write the complete query at once.

Instead, break the problem into smaller steps and build the query logically.

📌 1. Understand the Requirement

First, identify exactly what the question wants as the final output.

Example:

«Find the total revenue generated by each product.»

You need to display:

👉 Product
👉 Total Revenue

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📌 2. Identify the Required Table(s)

Find where the required data is stored.

Suppose you have:

"orders"

- product_id
- quantity
- price
- order_date

The required information is available in the "orders" table.

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📌 3. Identify the Required Columns

Don't select unnecessary columns.

For total revenue, you need:

"quantity × price"

Therefore:

👉 "product_id"
👉 "quantity"
👉 "price"

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📌 4. Check for Filtering

Ask yourself:

«Do I need only specific rows?»

For example:

«Find the revenue generated during 2026.»

You would need:

WHERE order_date >= '2026-01-01'

💡 WHERE filters rows before aggregation.

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📌 5. Look for GROUP BY Keywords

Words such as:

• Each
• Per
• By
• For every

often indicate that "GROUP BY" is required.

Example:

«Find revenue for each product.»

GROUP BY product_id

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📌 6. Choose the Correct Aggregate Function

Look for keywords in the question:

Total → "SUM()"
Average → "AVG()"
Number of records → "COUNT()"
Highest → "MAX()"
Lowest → "MIN()"

For total revenue:

SUM(quantity * price)

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📌 7. Build the Query Step by Step

Don't jump directly to the final query.

Step 1 — Select the required column

SELECT product_id
FROM orders;

Step 2 — Add the calculation

SELECT
product_id,
SUM(quantity * price) AS total_revenue
FROM orders;

Step 3 — Add grouping

SELECT
product_id,
SUM(quantity * price) AS total_revenue
FROM orders
GROUP BY product_id;

Now the basic query is complete. ✅

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📌 8. Know When to Use HAVING

Suppose the question is:

«Find products whose total revenue is greater than ₹50,000.»

Since the condition is applied to an aggregate result, use "HAVING":

SELECT
product_id,
SUM(quantity * price) AS total_revenue
FROM orders
GROUP BY product_id
HAVING SUM(quantity * price) > 50000;

🧠 Remember:

"WHERE" → filters individual rows
"HAVING" → filters grouped results

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📌 9. Check Whether a JOIN Is Required

Suppose the question asks:

«Display product names along with their total revenue.»

You have:

"products"

- product_id
- product_name

"orders"

- product_id
- quantity
- price

You need to connect the two tables:

SELECT
p.product_name,
SUM(o.quantity * o.price) AS total_revenue
FROM products p
JOIN orders o
ON p.product_id = o.product_id
GROUP BY p.product_name;

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📌 10. Check for Advanced SQL Concepts

Some problems require more than basic clauses.

Before finalizing your query, ask:

👉 Do I need a "JOIN"?
👉 Do I need "GROUP BY"?
👉 Do I need "HAVING"?
👉 Do I need "ORDER BY"?
👉 Do I need "LIMIT"?
👉 Do I need a subquery?
👉 Would a CTE make the query easier?
👉 Do I need a window function?

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🧠 SQL Problem-Solving Framework

Whenever you get a SQL question, think in this order:

Understand → Tables → Columns → JOIN → WHERE → GROUP BY → Aggregate → HAVING → ORDER BY → Window Functions → Validate

⚡ Don't memorize SQL queries. Learn how to build them logically.

📌 Save this framework for your next SQL interview.