SQL Interview Questions Cheat Sheet: 21 Patterns You Must Know

If you're preparing for a SQL interview, you don't need hundreds of random questions.

Most real interviews keep repeating the same patterns:

• Filtering and sorting on realistic tables
• Joining 2–4 tables correctly
• Aggregations with GROUP BY and HAVING
• Window functions for rankings and running totals
• Date logic and time ranges
• Edge cases around NULLs and duplicates

In this guide we'll walk through the most common patterns interviewers use to test whether you're ready for the job.

1. Simple filter with ORDER BY

"Give me active customers from India, sorted by signup date"

Interviewers want to see:

• Clean WHERE conditions
• Correct ordering (often DESC on recent activity)
• Filters that match the business wording

sql
SELECT customer_id, name, country, signup_date
FROM customers
WHERE status = 'active'
  AND country = 'India'
ORDER BY signup_date DESC;

💡 Tip: Always confirm whether status is a string, enum, or derived flag from another table.

2. INNER JOIN with clear join keys

"List customers and their total order count"

The goal is to check if you:

• Join on the right keys
• Aggregate on the right table
• Avoid double-counting

sql
SELECT
  c.customer_id,
  c.name,
  COUNT(o.order_id) AS total_orders
FROM customers c
JOIN orders o
  ON c.customer_id = o.customer_id
GROUP BY c.customer_id, c.name;

Common mistakes:

• Missing GROUP BY columns
• Joining on non-unique columns and inflating counts

3. LEFT JOIN for optional relationships

"Show all customers and how many orders they placed (including 0)"

This is almost the same as the previous question, but requires a LEFT JOIN:

sql
SELECT
  c.customer_id,
  c.name,
  COUNT(o.order_id) AS total_orders
FROM customers c
LEFT JOIN orders o
  ON c.customer_id = o.customer_id
GROUP BY c.customer_id, c.name;

💡 Pattern: When the question says "including users who never..." or "even if there are no orders", think LEFT JOIN.

4. GROUP BY with HAVING

"Return products that have been ordered more than 100 times"

sql
SELECT
  product_id,
  COUNT(*) AS order_count
FROM order_items
GROUP BY product_id
HAVING COUNT(*) > 100;

Use HAVING when you filter on aggregated values.

5. Top-N per group with window functions

"For each country, find the top 3 customers by total spend"

This is a classic window function pattern:

sql
WITH customer_spend AS (
  SELECT
    c.customer_id,
    c.country,
    SUM(o.amount) AS total_spend
  FROM customers c
  JOIN orders o
    ON c.customer_id = o.customer_id
  GROUP BY c.customer_id, c.country
),
ranked AS (
  SELECT
    customer_id,
    country,
    total_spend,
    DENSE_RANK() OVER (
      PARTITION BY country
      ORDER BY total_spend DESC
    ) AS rnk
  FROM customer_spend
)
SELECT *
FROM ranked
WHERE rnk <= 3;

If you can confidently explain what PARTITION BY and ORDER BY do here, you're ahead of most candidates.

6. Running totals (cumulative sums)

"Show daily revenue and cumulative revenue over time"

sql
WITH daily_revenue AS (
  SELECT
    order_date,
    SUM(amount) AS revenue
  FROM orders
  GROUP BY order_date
)
SELECT
  order_date,
  revenue,
  SUM(revenue) OVER (
    ORDER BY order_date
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
  ) AS cumulative_revenue
FROM daily_revenue
ORDER BY order_date;

Interviewers use this to test your comfort with window frames.

7. Date range and time filtering

"Count weekly active users for the last 4 weeks"

You'll often see:

• DATE_TRUNC in Postgres
• DATE_FORMAT or DATE_ADD in MySQL

sql
SELECT
  DATE_TRUNC('week', event_time) AS week_start,
  COUNT(DISTINCT user_id) AS wau
FROM user_events
WHERE event_time >= NOW() - INTERVAL '28 days'
GROUP BY DATE_TRUNC('week', event_time)
ORDER BY week_start;

8. Handling NULLs correctly

Questions that mention "missing", "unknown", or "no value" are testing how you handle NULL.

• Use IS NULL / IS NOT NULL
• Be careful with = and <> (they ignore NULLs)

sql
SELECT *
FROM customers
WHERE email IS NULL;

9. Real-world join + filter interview question

"Find the top 5 products by revenue in the last 30 days, excluding cancelled orders"

This combines several patterns:

sql
SELECT
  oi.product_id,
  SUM(oi.quantity * oi.unit_price) AS revenue
FROM order_items oi
JOIN orders o
  ON oi.order_id = o.order_id
WHERE o.status <> 'cancelled'
  AND o.order_date >= CURRENT_DATE - INTERVAL '30 days'
GROUP BY oi.product_id
ORDER BY revenue DESC
LIMIT 5;

If this query feels natural to you, you're very close to being interview-ready.

10. How to practice these patterns the right way

Reading solutions is not enough. Interviewers care about how you think, not just the final query.

When you practice:

1. Start from the business question and restate it in your own words.
2. Identify the tables and join keys you'll need.
3. Decide whether you need aggregations, window functions, or both.
4. Write the query step by step instead of trying to be clever in one shot.

If you want structured practice with problems that mirror real interview questions from FAANG and top startups, check out the SQL Interview Preparation Kit on TheQueryLab – it combines:

• Realistic schemas
• Pattern-based questions
• Execution environment you can use directly in your browser

So you don't just memorize solutions – you build intuition.