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What is a Composite Key? Definition A composite key is a key made of two or more columns that together uniquely identify a row, while each column alone is not sufficient. It can be: a composite primary key a composite … Continue reading →

1️⃣ Candidate Key Definition A candidate key is any minimal set of columns that can uniquely identify a row in a table. Properties Uniqueness Minimal (no redundant columns) There can be multiple candidate keys per table Example Candidate keys: id … Continue reading →

When should you use a relational database? Use a relational DB by default unless you have a strong reason not to. ✅ Use a relational DB when: 1️⃣ You need strong consistency & correctness Financial transactions Orders, payments, balances Inventory, … Continue reading →

1️⃣ Relational Databases (SQL) Model Tables (rows & columns) Fixed schema Relations via foreign keys ACID transactions Examples PostgreSQL MySQL Oracle SQL Server Best for Business data Financial systems Strong consistency Complex queries & joins Interview keyword “Strong consistency, relational … Continue reading →

What is SQL? SQL (Structured Query Language) is a standard language used to: Query data from relational databases Modify data (INSERT, UPDATE, DELETE) Define database structure (tables, indexes, constraints) Control access (GRANT, REVOKE) It operates on data stored in tables … Continue reading →

As a senior backend engineer, you’re expected to classify operators, not just list random ones. Short interview answer:SQL operators are used in expressions to compare values, filter rows, perform calculations, and combine conditions. They include arithmetic, comparison, logical, set, and … Continue reading →

Introduction In JPA and Hibernate, repository methods can return either List<DTO> or Stream<DTO>.At first glance, this looks like a minor API choice. In reality, it reflects two fundamentally different data processing models with serious implications for memory usage, GC behavior, … Continue reading →

SQL (Structured Query Language) is a declarative language used to define, query, and manipulate data in relational databases. Interview-friendly one-liner:SQL is a declarative language for working with relational data: you describe what data you want, not how to get it. … Continue reading →

Changing database credentials or the JDBC URL during a migration is riskier than it sounds. The impact depends on when and how you change them, but the common failure mode is schema history divergence and partial application. Here’s how I’d … Continue reading →

Common security concerns with migration tools aren’t about “SQL is scary” — they’re about credentials, privileges, supply chain, and auditability. Here’s what I’d expect you to cover at a senior interview level. 1) Over-privileged migration accounts Risk: Migration user has … Continue reading →