Question 1

What is the CASE expression?

Accepted Answer

**Target Audience:** Beginners who can write SELECT, WHERE and GROUP BY and need "if this, then that" logic inside a query.
**Difficulty:** Easy
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, which runs the examples on this page) and PostgreSQL 16.15; SQL Server and MySQL notes are marked with their source.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output and error

Question 2

How do you find duplicate rows in a table?

Accepted Answer

**Target Audience:** Beginners who know `SELECT` and want the standard duplicate-finding pattern, and candidates who want the details interviewers probe (NULLs, case, collation).
**Difficulty:** Easy
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16.15, both run for this page. MySQL behaviour is cited from its manual.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so

Question 3

When would you use a transaction explicitly?

Accepted Answer

**Target Audience:** Beginners who know `INSERT` and `UPDATE` and want to know when wrapping them in `BEGIN ... COMMIT` matters, and candidates who want concrete scenarios rather than a definition.
**Difficulty:** Easy
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16.15, both run for this page. Notes on MySQL and drivers are cited.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Inte

Question 4

What is a CROSS JOIN and when is it useful?

Accepted Answer

**Target Audience:** Beginners who have seen INNER and LEFT JOIN and want to know what the "join with no condition" is for, plus candidates who want the engine details.
**Difficulty:** Easy
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16.15, both run for this page. MySQL behaviour is cited from its manual.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also

Question 5

What do COALESCE and NULLIF do?

Accepted Answer

**Target Audience:** Beginners who have met NULL and want the two tools that tame it, and candidates who need the engine details.
**Difficulty:** Easy
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16. SQL Server and MySQL variants are cited from their documentation.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you learn the words an interviewer expects. Every re

Question 6

How does GROUP BY work with aggregate functions?

Accepted Answer

**Target Audience:** Beginners writing their first "total per category" query, and candidates who need the single-value rule and the engine differences.
**Difficulty:** Medium
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16. MySQL behaviour is cited from its manual.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you learn the words an interviewer expects. Every r

Question 7

How does SQL handle NULL values?

Accepted Answer

**Target Audience:** Beginners who know SELECT and WHERE, and anyone who has been surprised by a query that "lost" rows.
**Difficulty:** Medium
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16. MySQL, SQL Server and Oracle differences are cited where they matter.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you learn the words an interviewer expects. Every resul

Question 8

FOR UPDATE vs FOR UPDATE NOWAIT vs FOR UPDATE SKIP LOCKED — which to use when?

Accepted Answer

**Target Audience:** Developers who know basic transactions and want to understand locking reads from zero; candidates who need a crisp "which one when" answer.
**Difficulty:** Medium
**Engines:** PostgreSQL 16.15 (two real `psql` sessions for every locking behaviour, the verifier for the examples), SQLite via sql.js 1.10.2 for the runnable query and to confirm it has no `FOR UPDATE`. MySQL and SQL Server facts are cited, not run.

> **How to read this doc:** Every idea is explained in plain lan

Question 9

What are the common aggregate functions?

Accepted Answer

**Target Audience:** Beginners writing their first reports, and candidates who want the NULL, empty-set and type rules an interviewer probes.
**Difficulty:** Medium
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16; one MySQL fact is cited from its manual.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you learn the words an interviewer expects. Every result shown

Question 10

How do you pivot rows into columns?

Accepted Answer

**Target Audience:** Anyone building reports in SQL, from beginners who know `GROUP BY` to candidates who will be asked about dynamic pivots and engine-specific `PIVOT` syntax.
**Difficulty:** Hard
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16.15 with the `tablefunc` 1.0 extension, both run for this page. SQL Server and Oracle syntax is cited, not run.

> **How to read this doc:** Every idea is explained in plain language first, t

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