Interview questions in Databases & Architecture
What is the difference between primary, foreign, and unique keys?
This tests relational integrity basics. Answer: primary keys identify rows, foreign keys reference tables, and unique keys are alternate candidates. Red flag: saying unique keys are just for indexing or omitting a non-PK example like email.
Explain ACID properties and why they matter for banking or e-commerce
Define each as a failure-handling guarantee; show how partial commits cause double-spending.
What is the difference between DDL and DML in SQL?
DDL shapes schema with CREATE or ALTER; DML handles row-level data with SELECT, INSERT, UPDATE, or DELETE.
Describe 1NF, 2NF, 3NF, normalization's purpose, and its performance trade-off.
1NF atomic values; 2NF no partial dependencies; 3NF no transitive dependencies; prevents update anomalies but adds join overhead.
Explain database indexes, the classic data structure, and write-heavy trade-offs
Tests the read-write trade-off of indexing. A strong answer names B-Trees, explains they avoid full scans, and notes that inserts, updates, and deletes must update the index, adding write amplification and storage cost. Red flag: claiming indexes are free.
Find users who never placed an order and explain JOIN choice
This tests SQL anti-joins and NULL semantics. A strong answer uses LEFT JOIN with IS NULL or NOT EXISTS, explains why NOT IN is risky with NULLs, and why NOT EXISTS is preferred. Red flag: using INNER JOIN or ignoring NULLs.
What is the difference between WHERE and HAVING in SQL?
Tests SQL execution order and aggregation. A strong answer states WHERE filters rows before grouping, HAVING filters groups after aggregation, and gives an aggregate example that WHERE cannot evaluate.
Read Committed versus Serializable isolation levels
Name the four levels, map each anomaly (dirty read, non-repeatable read, phantom) to the level that blocks it.
Diagnosing and fixing the N+1 query problem
Define the 1 parent plus N child queries, fix via JOIN or batched IN, and ORM eager loading.
When to intentionally denormalize a schema
Identify read-heavy join cost, duplicate or precompute data, and own the consistency burden.
Normalizing a flat orders table to 3NF
Split repeating data, remove partial dependencies, remove transitive dependencies, define keys.
Modeling one-to-many versus many-to-many relationships
One-to-many uses a foreign key on the many side; many-to-many needs a junction table with two foreign keys.
Adjacency List versus Nested Set for hierarchies
Adjacency list is simple writes but recursive reads; nested set is fast subtree reads but costly writes.
3NF versus BCNF and the overlapping-key gap
BCNF requires every determinant be a superkey; 3NF allows exceptions for prime attributes.
Polymorphic associations and referential integrity
A single column can't FK two tables, so integrity is unenforced; alternatives use exclusive arcs or per-type tables.
Relational versus wide-column for a news feed
Relational gives flexible joins but read-time fan-out; Cassandra precomputes per-user feed rows for fast writes-side fan-out.
Shard key impact on uniqueness and cross-shard lookups
Uniqueness and FKs hold only within a shard; non-shard-key lookups need scatter-gather or a secondary index.
The ACID properties of transactions
Define Atomicity, Consistency, Isolation, Durability and why each matters.
The lost update anomaly explained
Both transactions read the same value, each adds one, the second overwrite erases the first.
Database deadlocks and how engines resolve them
Define a deadlock as mutual waiting on locks, name detection plus victim rollback, and prevention by consistent lock ordering.
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