SQL interview questions

10 questions with answers, grouped by topic.

Query basics

SELECT, WHERE, ORDER BY and how a query is actually executed.

1

What is the logical processing order of a SELECT statement?intermediate

Joins & sets

Combining tables without silently changing your row count.

3

Why might a LEFT JOIN behave like an INNER JOIN?intermediate How do you find rows in one table with no match in another?beginner What is the difference between UNION and UNION ALL?beginner

Aggregation

GROUP BY, HAVING, and the mistakes that produce wrong totals.

2

What is the difference between WHERE and HAVING?beginner How do you count things conditionally in one query?intermediate

Window functions

Ranking, running totals and period-over-period.

2

Explain the difference between RANK, DENSE_RANK and ROW_NUMBER.intermediate How would you calculate month-over-month growth?intermediate

Structure & performance

Subqueries, CTEs, indexes and reading a plan.

2

What causes a query to ignore an index?advanced What is a CTE and when would you use one?intermediate