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 model, declarative SQL”
2️⃣ NoSQL Databases (Non-Relational)
Designed to solve scalability, flexibility, and performance problems where SQL struggles.
2.1 Key–Value Stores
- Data:
key → value - Extremely fast
- Simple access pattern
Examples
- Redis
- DynamoDB
Use cases
- Caching
- Sessions
- Rate limiting
2.2 Document Databases
- JSON / BSON documents
- Flexible schema
- Nested structures
Examples
- MongoDB
- CouchDB
Use cases
- User profiles
- Content management
- Rapid schema evolution
2.3 Wide-Column (Column-Family)
- Rows with dynamic columns
- Optimized for massive scale
Examples
- Cassandra
- HBase
Use cases
- Time-series
- Event data
- Write-heavy systems
2.4 Graph Databases
- Nodes + edges + properties
- Relationship-first model
Examples
- Neo4j
- Amazon Neptune
Use cases
- Social networks
- Recommendation engines
- Fraud detection
3️⃣ Columnar / Analytical Databases (OLAP)
Model
- Data stored by column, not row
- Optimized for aggregation
- Read-heavy
Examples
- ClickHouse
- Snowflake
- BigQuery
Use cases
- Analytics
- Reporting
- BI dashboards
Interview keyword
“OLTP vs OLAP separation”
4️⃣ Time-Series Databases
Model
- Timestamped data
- High write throughput
- Retention policies
Examples
- InfluxDB
- TimescaleDB
Use cases
- Metrics
- Monitoring
- IoT
5️⃣ Search Engines (Search Databases)
Model
- Inverted indexes
- Full-text search
- Scoring & relevance
Examples
- Elasticsearch
- OpenSearch
Use cases
- Search
- Log aggregation
- Observability
⚠️ Interview trap
Not a replacement for transactional DBs.
6️⃣ In-Memory Databases
Model
- Data stored primarily in RAM
- Extremely low latency
Examples
- Redis
- Memcached
Use cases
- Caching
- Distributed locks
- Queues
7️⃣ NewSQL / Distributed SQL
Model
- SQL + ACID
- Horizontal scalability
- Cloud-native
Examples
- CockroachDB
- YugabyteDB
Use cases
- Global systems
- Strong consistency + scale
Senior-level summary (perfect spoken answer)
Databases can be broadly classified into relational SQL databases and several NoSQL categories like key-value, document, column-family, and graph databases. Additionally, there are specialized systems such as columnar OLAP databases, time-series databases, search engines, and distributed SQL systems. The choice depends on consistency requirements, query patterns, and scalability needs.