WITH sales_summary AS (
    SELECT customer_id,
           SUM(amount) AS total_sales
    FROM sales
    GROUP BY customer_id
)
SELECT *
FROM sales_summary
WHERE total_sales > 10000;

This makes your SQL easier to read and debug.

📌 16. Don't Memorize Interview Queries

Instead of memorizing:

"Query to find the second-highest salary"

Understand the underlying concept:

Ranking → Ordering → Selecting the required rank.

This allows you to solve variations of the same problem.

📌 17. Practice Real Business Scenarios

Don't practice only basic employee tables.

Try problems involving:

Sales

Customers

Orders

Transactions

Revenue

Products

Employee retention

Customer churn

Fraud detection

This will make your SQL much more practical.

📌 18. Always Validate Your Output

After writing a query, ask yourself:

Is the row count correct?

Are duplicates present?

Are NULLs handled?

Are totals correct?

Did the JOIN produce the expected records?

Does the result actually answer the business question?

Writing a query is only half the job. Validating the result is equally important.

📌 19. Understand SQL Execution Order

A simplified logical execution order is:

FROM
↓
WHERE
↓
GROUP BY
↓
HAVING
↓
SELECT
↓
ORDER BY
↓
LIMIT

Understanding this will help you solve many tricky SQL interview questions.

📌 20. Practice Consistently

You don't need to solve 100 questions in one day.

Solve 5–10 SQL problems every day and gradually move from:

Basic Queries
      ↓
Filtering
      ↓
Aggregations
      ↓
JOINs
      ↓
CASE WHEN
      ↓
Subqueries
      ↓
CTEs
      ↓
Window Functions
      ↓
Advanced SQL

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