How are relationships between tables implemented in a database when sharding is used?

When sharding a database, data is divided into parts (shards) stored on different servers. This complicates the implementation of relationships between tables, especially if related data is in different shards.

Main approaches:

• Denormalization of data: instead of complex JOIN queries, data is duplicated in each shard to reduce cross-shard relationships.
• Local relationships: design sharding so that related data falls into the same shard (for example, by user key), allowing relationships to be established locally.
• Use of external services: to combine data from different shards, application-level services can be used.

Thus, direct relationships between tables in sharding are often replaced by architectural solutions that minimize cross-shard dependencies.