Question 1

What are transaction isolation levels?

Accepted Answer

**Target Audience:** Beginners who know what a transaction is and want to understand what "isolation level" means, and candidates who need the anomaly names, the four standard levels and the real engine defaults.
**Difficulty:** Medium
**Engines:** PostgreSQL 16.15 (all two-session timelines were run on it with two concurrent `psql` sessions) and SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples use). MySQL, SQL Server and Oracle facts are cited from their documentation. The Postgre

Question 2

What is the difference between OLTP and OLAP databases?

Accepted Answer

**Target Audience:** Beginners who hear "OLTP" and "OLAP" in architecture discussions, and candidates who need to explain why companies run a separate data warehouse.
**Difficulty:** Medium
**Engines:** The SQL examples run on SQLite 3.45.0 (sql.js 1.10.2, the engine this page uses) and returned the same rows on PostgreSQL 16.15; the plan comparison ran on PostgreSQL 16.15. Facts about warehouse products are cited from their documentation, not run here.

> **How to read this doc:** Every idea is

Question 3

UPSERT with INSERT … ON CONFLICT — how do you insert or update atomically without races?

Accepted Answer

**Target Audience:** Backend developers who write "create or update" code and candidates who will be asked why the naive version is unsafe.
**Difficulty:** Medium
**Engines:** PostgreSQL 16.15 (concurrency tests, the batch example) and SQLite 3.45.0 through sql.js 1.10.2 (the runnable examples on this page). MySQL and SQL Server syntax notes are cited from their documentation, not run.

> **How to read this doc:** Every idea is explained in plain language first. Then a `📌 Interview term:` callo

Question 4

What is the difference between DELETE, TRUNCATE, and DROP?

Accepted Answer

**Target Audience:** Beginners who need to remove data safely, and candidates preparing for the engine-specific follow-ups (transactions, locks, identity, space).
**Difficulty:** Medium
**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, including a two-session test on PostgreSQL. MySQL and SQL Server behaviour is cited from their manuals.

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

Question 5

SKIP LOCKED for job queues — how do you build a concurrent queue without race conditions?

Accepted Answer

**Target Audience:** Backend developers who want a reliable job queue on PostgreSQL without a broker, and candidates asked to design one in an interview.
**Difficulty:** Hard
**Engines:** PostgreSQL 16.15 (two real `psql` sessions for every race, the verifier for the claim and plan) and SQLite via sql.js 1.10.2 for the runnable claim and reaper. MySQL facts are cited, not run.

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

SQLDECODED.

Medium

What are transaction isolation levels?

What is the difference between OLTP and OLAP databases?

UPSERT with INSERT … ON CONFLICT — how do you insert or update atomically without races?

What is the difference between DELETE, TRUNCATE, and DROP?

Hard

SKIP LOCKED for job queues — how do you build a concurrent queue without race conditions?