Top 25 SQL Interview Questions for Experienced Developers
Top 25 SQL Interview Questions for Experienced Developers Work through these 15 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
When and why use CQRS (Command Query Responsibility Segregation)?
E-commerce: write Order aggregate to PostgreSQL; project OrderSummaryView to Elasticsearch for search/filter dashboard. Accept projection lag. Not for simple User CRUD. Combine...
A REST API is slow only in production, not locally. How do you debug it?
Enable Micrometer timers and Hibernate statistics or datasource-proxy in staging with prod-like data. Check Hikari `connections-active`, JVM GC pauses, and Feign/RestClient...
What is the difference between ArrayList and LinkedList?
ArrayList: dynamic array, great for get(i) and iteration, amortized append. LinkedList: node chain, costly random access, only wins for frequent insert/remove in middle with...
What are text blocks and how do they improve Java code?
String query = """ SELECT u.id, u.name FROM users u WHERE u.active = true """; — readable without \n concatenation. Use formatted for parameters carefully — prefer...
Best practices for Spring Data pagination with Pageable?
Controller: @PageableDefault(size=20, sort="createdAt", direction=DESC) Pageable p. Cap size at 100. Admin reports: keyset pagination on id > lastId for deep pages — OFFSET...
What is the N+1 query problem and how do you fix it?
Loading 100 orders then accessing each order's items triggers 101 queries. Fix with JOIN FETCH in JPQL, @EntityGraph on repository method, or a DTO query selecting only...
What is Hibernate second-level cache and when enable it?
Enable for ProductCategory, Currency — READ_ONLY, rarely changed. spring.jpa.properties.hibernate.cache.use_second_level_cache=true with Caffeine region factory. Avoid on...
How implement multi-tenancy in Spring Security context?
JWT includes tenant_id — converter sets TenantContextHolder. Base entity @Column tenantId validated on save. Repository methods always include tenant predicate — or Hibernate...
Saga pattern — choreography vs orchestration?
Order saga: reserve inventory → charge payment → ship. Failure at payment triggers release inventory compensation. Orchestration clearer for complex flows with visibility....
Explain the transactional outbox pattern.
OrderService @Transactional saves Order and OutboxEvent(type=ORDER_CREATED, payload=json). OutboxProcessor @Scheduled polls FOR UPDATE SKIP LOCKED, sends to Kafka, updates...
Database sharding strategies for high-scale Java services?
Shard by tenant_id mod 32 — RoutingDataSource lookup. User-scoped queries single shard. Global admin reports from CQRS read replica or Elasticsearch. Plan shard key early —...
How does @Transactional work?
Spring creates a proxy around @Transactional beans. Calling createOrder() from another bean starts a transaction on the thread-bound EntityManager; commit on success, rollback...
How do Spring application events work with @EventListener?
OrderService publishes OrderCreatedEvent after save. EmailListener @EventListener sends confirmation. @TransactionalEventListener(AFTER_COMMIT) prevents email on rolled-back...
When use @Query native SQL vs JPQL in Spring Data JPA?
JPQL for standard entity fetches and portable logic. Native for PostgreSQL-specific jsonb queries, recursive CTEs, or performance-tuned index hints after EXPLAIN. Interface...
How do Flyway/Liquibase integrate with Spring Boot JPA?
Prod: spring.jpa.hibernate.ddl-auto=validate, Flyway V1__init.sql through Vn. CI: @Testcontainers PostgreSQL + Flyway migrate + integration tests. Never rename column without...
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