What are the advantages of NoSQL databases over relational ones? What is the CAP theorem?

Advantages of NoSQL databases over relational ones:

• Flexible data schema: NoSQL allows storing data without a strict schema, which is convenient for rapidly changing or unstructured data.
• Scalability: NoSQL databases are often easier to scale horizontally (by adding servers).
• High performance with large volumes of data and distributed systems.
• Support for various data models: documents, graphs, key-value, column stores.

CAP theorem: The CAP theorem states that in a distributed system, it is impossible to guarantee all three properties simultaneously:

• Consistency: all nodes see the same data at the same time.
• Availability: every request receives a response (successful or error).
• Partition tolerance: the system continues to operate despite network partitions.

In real systems, you have to choose two of the three properties, which affects the architecture and behavior of the database.