Redis keeps its data in memory as keys holding data structures, while a relational database keeps tables on disk and answers SQL queries. Redis is far faster for simple reads and writes, and a relational database gives you joins, a fixed schema and full transactions.
Aspect
Redis
Relational database
Storage
In memory, so data must fit in RAM
Disk-based
Data model
Key-value, NoSQL
Relational tables
Schema
Schema-less
Fixed schema
ACID
Limited: MULTI and EXEC have no rollback
Full ACID compliance
Performance
Extremely fast, microseconds
Slower, milliseconds
Persistence
Optional, as snapshots or a log of writes
Always persistent
Query language
Commands
SQL
Relationships
No joins
Complex relationships
Redis suits caching, sessions and real-time work such as counters and leaderboards. A relational database suits complex business logic and data with many relationships. Many systems use both. The relational database stays the source of truth, and Redis holds a fast copy of the data read most often.
Rewriting in plainer words…
This answer doesn't lend itself to a diagram - it reads best . No credits were charged.
