Interview Q&A / SQL & DBMS

1. How do you find rows in one table that have no matching row in another?

   Use LEFT JOIN and keep rows where the right table's key IS NULL, or use NOT EXISTS. Both are safer than NOT IN, which fails when the subquery returns a NULL.

   IntermediateQuick answerCoding1 min read
2. Write a query to calculate a running total of daily sales.

   Use SUM(amount) OVER (ORDER BY day) to get a cumulative sum. Add PARTITION BY to restart the total per group, and use ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW to be explicit.

   IntermediateQuick answerCoding1 min read
3. Write a query to find the highest-paid employee in each department.

   Compute MAX(salary) per department in a subquery and join back, or use RANK() OVER (PARTITION BY dept ORDER BY salary DESC) and keep rank 1. Both return all tied employees.

   IntermediateQuick answerCoding1 min read