SQL Developer Interview Questions UK: 8 Real Questions and What Interviewers Want

Most SQL Developer interviews in the UK follow a pattern, and knowing it beats guessing. Below are eight questions from our bank of 16, each with a note on what the interviewer is really after.

Eight SQL Developer interview questions, and what the interviewer is thinking

1. What is the difference between a clustered and a non-clustered index in SQL Server, and how do you decide what to index?

Why they ask: Indexing is the most frequently tested SQL Server performance topic.

2. How would you diagnose and fix a slow-running stored procedure in SQL Server?

Why they ask: Performance tuning is the core skill separating mid-level from junior SQL developers.

3. Explain the different types of SQL joins and when you would use each, including how NULLs behave.

Why they ask: Joins are fundamental and used in almost every interview to test basic accuracy.

4. What are window functions, and can you give examples of where you have used them?

Why they ask: Window functions separate competent T-SQL developers from those writing clumsy subqueries.

5. What are CTEs, temp tables and table variables, and how do you choose between them?

Why they ask: This tests practical judgement on structuring and optimising T-SQL.

6. A report that normally runs in seconds has suddenly taken several minutes in production. How do you investigate?

Why they ask: Production troubleshooting under pressure is part of the day job.

7. Tell me about a time you significantly improved the performance of a query, procedure or ETL process.

Why they ask: They want evidence that you can deliver measurable tuning results.

8. What does a SQL Developer do day to day, and what makes someone good at the job?

Why they ask: They want to check your understanding of the role and your attitude to quality.

A sample answer to question 1

What is the difference between a clustered and a non-clustered index in SQL Server, and how do you decide what to index?

A clustered index defines the physical order of the data in the table, so there can only be one, and the leaf level is the data itself. A non-clustered index is a separate structure with its own key columns, which points back to the clustered key or a row identifier. To decide what to index, I look at real workload rather than guessing: query plans, Query Store, and the missing index DMVs, bearing in mind their suggestions are only hints. I'd choose key columns based on WHERE, JOIN and ORDER BY usage, put equality columns before range columns, and add INCLUDE columns to make the index covering and avoid key lookups. I balance this against write cost, because every index slows inserts and updates and takes storage. I'd review unused and duplicate indexes with sys.dm_db_index_usage_stats, check fragmentation and statistics, and generally keep the clustered key narrow, unique and ever-increasing.

Common mistake to avoid: Indexing every column; Following missing-index hints blindly

Want the rest?

Full model answers, key points and common mistakes for all 16 questions are on the SQL Developer interview questions page. The first five are free, no sign-up needed.