LeetCode SQL Problems Guide: Patterns Interviewers Test

If you thought LeetCode SQL problems were only for data analysts and database engineers, think again. SQL coding rounds have become a standard part of the interview loop at companies like Meta, Amazon, Google, and virtually every mid-to-large tech company. Backend engineers, full-stack developers, and even frontend engineers are now expected to demonstrate SQL competency.

The reason is simple: every application touches a database, and interviewers want to know you can write efficient queries without leaning on an ORM for everything. Unlike algorithmic coding rounds where you might never use a trie in production, SQL skills translate directly to day-to-day work.

The good news is that LeetCode SQL problems follow predictable patterns. Once you recognize what concept a question is testing — whether it is a JOIN, a window function, or a subquery — the solution often writes itself. This guide breaks down every pattern you need, organized by difficulty and frequency.

SQL interviews are not about obscure database trivia. They focus on a small set of core concepts that come up again and again. If you can handle these confidently, you can solve the vast majority of leetcode sql problems you will encounter.

JOINs are the foundation. You need to understand INNER JOIN, LEFT JOIN, and when to use each. GROUP BY with HAVING lets you aggregate and filter groups — a pattern that appears in nearly half of all SQL interview questions. Subqueries, both correlated and non-correlated, let you break complex problems into manageable steps.

Window functions are where most candidates separate themselves. RANK(), DENSE_RANK(), ROW_NUMBER(), and analytic functions like LAG() and LEAD() are tested frequently at senior levels. CTEs (Common Table Expressions) help you write readable, modular queries. Self-joins solve problems where you need to compare rows within the same table. And CASE WHEN expressions handle conditional logic directly in your queries.

If you are new to sql practice problems on LeetCode, start with the Easy tier. These problems test fundamental concepts — basic JOINs, simple filtering, and straightforward aggregation. They build the muscle memory you need before tackling anything harder.

Combine Two Tables (#175) is the classic starter. It asks you to LEFT JOIN two tables to display person names alongside their city and state. If you cannot solve this in under two minutes, spend more time on JOIN basics before moving forward. Second Highest Salary (#176) introduces subqueries and the LIMIT/OFFSET pattern — a question that appears in interviews far more often than its Easy label suggests.

Employees Earning More Than Their Managers (#181) is your introduction to self-joins. You join the Employee table to itself to compare each employee's salary with their manager's salary. Duplicate Emails (#182) tests GROUP BY and HAVING — find all emails that appear more than once in the Person table.

These four problems alone cover JOINs, subqueries, self-joins, and GROUP BY. Master them and you have a solid foundation for the sql coding interview patterns that follow.

Medium leetcode database questions are where interviews actually happen. These problems combine multiple concepts and require you to think through edge cases. If you can solve these consistently, you are well-prepared for most SQL rounds.

Rank Scores (#178) is the definitive window function problem. It asks you to rank scores in descending order with no gaps — the exact scenario that DENSE_RANK() was built for. This single problem teaches you more about window functions than any tutorial. Consecutive Numbers (#180) tests your ability to detect patterns across adjacent rows using self-joins or LAG/LEAD.

Department Highest Salary (#184) combines JOIN with GROUP BY and subqueries. You need to find the employee with the highest salary in each department — a pattern that maps directly to real business queries. Exchange Seats (#626) is a classic CASE WHEN problem that asks you to swap adjacent seat IDs. Tree Node (#608) tests your ability to classify nodes as Root, Inner, or Leaf using CASE WHEN with subqueries.

Focus your sql interview prep time here. These five problems cover window functions, self-joins, GROUP BY with subqueries, CASE WHEN logic, and multi-table JOINs — the exact combination you will see in real interviews.

Hard LeetCode SQL problems are reserved for senior and staff-level interviews, or companies that pride themselves on rigorous SQL rounds. These problems require combining multiple advanced concepts in a single query.

Trips and Users (#262) is widely considered the hardest SQL problem on LeetCode. It requires filtering by date range, joining with a users table to exclude banned users, and calculating cancellation rates using GROUP BY with CASE WHEN aggregation. Human Traffic of Stadium (#601) asks you to find three or more consecutive rows where attendance exceeds 100 — a challenging consecutive-sequence problem that can be solved with window functions and self-joins.

Department Top Three Salaries (#185) is the natural progression from #184. Instead of finding the single highest salary per department, you need the top three. This is where DENSE_RANK() OVER (PARTITION BY department ORDER BY salary DESC) becomes essential. Median Employee Salary (#569) is a premium problem that tests your ability to calculate medians — a concept that has no built-in SQL function and requires creative use of ROW_NUMBER() and COUNT().

Do not attempt these until you are comfortable with each individual concept. The difficulty comes from combining them, not from any single technique being unfamiliar.

Window functions are the single most important advanced SQL concept for interviews. They let you perform calculations across a set of rows related to the current row — without collapsing the result set like GROUP BY does. If there is one topic to master for your sql coding interview, this is it.

The three ranking functions — ROW_NUMBER(), RANK(), and DENSE_RANK() — are tested constantly. ROW_NUMBER() assigns a unique sequential integer to each row. RANK() assigns the same rank to ties but leaves gaps (1, 2, 2, 4). DENSE_RANK() assigns the same rank to ties with no gaps (1, 2, 2, 3). Knowing which to use and why is critical — interviewers will ask you to explain the difference.

Beyond ranking, window functions enable running totals with SUM() OVER (ORDER BY ...), moving averages with AVG() OVER (ROWS BETWEEN ... AND ...), and row comparisons with LAG() and LEAD(). The PARTITION BY clause lets you restart calculations within groups — for example, ranking employees within each department separately.

The syntax follows a consistent pattern: FUNCTION() OVER (PARTITION BY column ORDER BY column). Once you internalize this template, sql window functions interview questions become significantly more approachable. Practice writing the OVER clause from memory until it is automatic.

With over 200 sql leetcode problems available, it is easy to feel overwhelmed. The good news is that you do not need to solve all of them. A focused strategy of 30 to 40 problems, chosen to cover every major pattern, will prepare you for the vast majority of SQL interview rounds.

Start with the Easy JOIN problems (#175, #181, #182) to build confidence with basic table operations. Then move to GROUP BY and HAVING problems — these form the backbone of most real-world queries and interview questions alike. Once JOINs and aggregation feel automatic, tackle the medium window function problems (#178, #184) to build comfort with RANK, DENSE_RANK, and PARTITION BY.

Use LeetCode's built-in SQL editor to practice writing queries from scratch. Avoid the temptation to look at solutions before spending at least 15 to 20 minutes working through the problem yourself. When you do review solutions, focus on understanding why the approach works, not memorizing the exact query.

After each problem, review it with YeetCode flashcards to reinforce the pattern. Spaced repetition is especially effective for SQL because the patterns are finite and predictable — once you can recall the right approach from memory, the actual query writing becomes straightforward. Aim to solve two to three SQL problems per day, and within two weeks you will have covered every pattern that matters.