What types of databases exist?

Databases are classified by several features, the main types:

1. Relational (SQL-oriented):

   • Data is stored in tables with rows and columns.
   • Relationships between tables are defined by primary and foreign keys.
   • SQL language is used for queries.
   • Examples: PostgreSQL, MySQL, Oracle, SQL Server.

2. Non-relational (NoSQL):

   • Differ from relational with flexible schemas and horizontal scaling.
   • There are different data models:
     • Key-value: Redis, Memcached.
     • Document-oriented: MongoDB, Couchbase.
     • Wide-column: Cassandra, HBase.
     • Graph: Neo4j, ArangoDB.

3. In-Memory:

   • Data is stored directly in RAM for fast access.
   • Can be relational or NoSQL.
   • Examples: Redis, Memcached, SAP HANA.

4. Time Series:

   • Optimized for storing and analyzing data arriving in chronological order.
   • Examples: InfluxDB, TimescaleDB.

5. Multi-model:

   • Supports multiple data models simultaneously (e.g., document and graph).
   • Examples: ArangoDB, OrientDB.

6. Spatial:

   • Optimized for storing and querying data with geographic coordinates.
   • Often extensions of relational or NoSQL DBMS (e.g., PostGIS for PostgreSQL).

7. Master-Slave:

   • Architectural pattern for data replication. One node (master) handles writes, others (slaves) only read and synchronize with master. Used for read scaling and fault tolerance.

8. Sharded:

   • Data is distributed across multiple independent nodes (shards). Used for horizontal scaling and handling large data volumes.

Comparison table of relational and non-relational DBs (main differences):

Characteristic	Relational (SQL)	Non-relational (NoSQL)
Schema	Rigid, predefined	Flexible, dynamic
Scaling	Vertical (more difficult horizontal)	Horizontal (easier)
Data integrity	ACID (usually)	BASE (more often)
Data model	Tables	Key-value, document, graph, etc.
Query language	SQL	Various (API, specific)