SQL developer interview questions

SQL developer interview questions and answers

SQL comes up in almost every technical interview, and database roles go much deeper. These are the questions asked most, from basics to performance, with what each one checks.

What to revise before the interview

  • Joins, GROUP BY, subqueries and CTEs
  • Normalisation: 1NF, 2NF, 3NF
  • Primary, foreign and unique keys
  • Indexes: how they work, when they hurt
  • Transactions, ACID and isolation levels
  • Stored procedures, views and triggers
  • Reading execution plans
  • Deleting duplicates, finding gaps, top N per group

  1. 1.What is normalisation? Explain up to 3NF.

    What they're checking
    Database design fundamentals.
    How to answer
    Organising tables to reduce duplication. 1NF: atomic values, no repeating groups. 2NF: no partial dependency on part of a composite key. 3NF: no dependency of non-key columns on other non-key columns.

  2. 2.How does an index speed up a query, and when can it slow things down?

    What they're checking
    Performance understanding.
    How to answer
    An index (usually a B-tree) lets the database find rows without scanning the whole table. It slows inserts and updates, uses space, and won't help low-selectivity columns or functions applied to the column.

  3. 3.What does ACID mean?

    What they're checking
    Transaction basics.
    How to answer
    Atomicity (all or nothing), Consistency (rules hold), Isolation (concurrent transactions don't interfere), Durability (committed data survives crashes). Example: a bank transfer debits and credits together or not at all.

  4. 4.DELETE vs TRUNCATE vs DROP?

    What they're checking
    Common fresher question.
    How to answer
    DELETE removes chosen rows and can be rolled back in a transaction; TRUNCATE removes all rows quickly; DROP removes the table itself. Behaviour details vary slightly by database.

  5. 5.How do you remove duplicate rows but keep one?

    What they're checking
    A practical query problem.
    How to answer
    Use ROW_NUMBER() OVER (PARTITION BY the duplicate columns ORDER BY id) and delete rows where the number is greater than 1.

  6. 6.Find the top 3 earners in each department.

    What they're checking
    Window functions in action.
    How to answer
    DENSE_RANK() OVER (PARTITION BY department ORDER BY salary DESC) in a CTE, then filter rank <= 3. Mention RANK vs DENSE_RANK for ties.

  7. 7.A query is slow. How do you fix it?

    What they're checking
    Real troubleshooting.
    How to answer
    Read the execution plan, look for full scans and bad joins, add or adjust indexes, select only needed columns, avoid functions on indexed columns in WHERE, and check statistics are up to date.

  8. 8.What are isolation levels?

    What they're checking
    Deeper concurrency knowledge for experienced roles.
    How to answer
    Read uncommitted, read committed, repeatable read and serializable - each prevents more anomalies (dirty, non-repeatable and phantom reads) at a cost to concurrency.

  9. 9.View vs stored procedure?

    What they're checking
    Database object knowledge.
    How to answer
    A view is a saved query you select from; a stored procedure is saved logic that can take parameters, run several statements and modify data.

  10. 10.What is the difference between UNION and UNION ALL?

    What they're checking
    Basics and performance awareness.
    How to answer
    UNION removes duplicates (extra work); UNION ALL keeps all rows and is faster. Use UNION ALL unless you need de-duplication.

On a live call, Cuelyte AI hears questions like these and has an answer ready in about a second, built from your resume and notes. Coding interviews: how Cuelyte AI helps

Frequently asked questions

What SQL topics are asked most in interviews?

Joins, GROUP BY with HAVING, subqueries, window functions, normalisation, indexes and classic queries such as the Nth highest salary or deleting duplicates.

Which database should I practise on?

Any - MySQL, PostgreSQL or SQL Server. Standard SQL is mostly the same; just know the small differences in the one the job uses.

Are DBMS theory questions asked to freshers?

Yes, especially in campus drives: keys, normalisation, ACID, transactions and the difference between DBMS and RDBMS.

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