When a JD says strong SQL, it means these 8 things

1-4: group, join, buckets, dates

1. 01

   GROUP BY with HAVING

   WHERE filters rows. HAVING filters groups.

   SELECT city, SUM(amount) AS total
   FROM orders
   GROUP BY city
   HAVING SUM(amount) > 50000;

2. 02

   LEFT JOIN to find what's missing

   Customers who never ordered.

   SELECT c.name
   FROM customers c
   LEFT JOIN orders o
     ON c.customer_id = o.customer_id
   WHERE o.order_id IS NULL;

3. 03

   CASE WHEN buckets

   Turn amounts into bands a manager can read.

   SELECT order_id,
     CASE WHEN amount >= 5000 THEN 'High'
          WHEN amount >= 1000 THEN 'Mid'
          ELSE 'Low' END AS band
   FROM orders;

4. 04

   Month-wise numbers (MySQL)

   Monthly totals for a report.

   SELECT DATE_FORMAT(order_date, '%Y-%m') AS month,
          SUM(amount) AS total
   FROM orders
   GROUP BY month;

5-8: window functions, clean and readable

1. 05

   Top 3 per group with ROW_NUMBER

   Number rows inside each city, keep the first 3.

   SELECT * FROM (
     SELECT *, ROW_NUMBER() OVER (
       PARTITION BY city
       ORDER BY amount DESC) AS rn
     FROM orders) t
   WHERE rn <= 3;

2. 06

   Running total

   Cumulative sum by date.

   SELECT order_date, amount,
     SUM(amount) OVER (
       ORDER BY order_date) AS running
   FROM orders;

3. 07

   CTEs instead of nested mess

   Name the step, then use it.

   WITH city_total AS (
     SELECT city, SUM(amount) AS t
     FROM orders GROUP BY city)
   SELECT * FROM city_total
   WHERE t > 50000;

4. 08

   Duplicates and NULLs

   Find repeats, and stop NULLs breaking totals.

   SELECT email, COUNT(*) AS n
   FROM customers
   GROUP BY email
   HAVING COUNT(*) > 1;
   
   -- COALESCE(amount, 0) for NULLs

JD line, decoded

Practise these on real tables in the free SQL playground, then check your answers against the 8 things interviewers check.