Select the Redis logical database having the specified zero-based numeric index.
New connections always use the database 0.

Selectable Redis databases are a form of namespacing: all databases are still
persisted in the same RDB / AOF file. However different databases can have keys
with the same name, and commands like `FLUSHDB`, `SWAPDB` or `RANDOMKEY` work on
specific databases.

In practical terms, Redis databases should be used to separate different keys
belonging to the same application (if needed), and not to use a single Redis
instance for multiple unrelated applications.

When using Redis Cluster, the `SELECT` command cannot be used, since Redis
Cluster only supports database zero. In the case of a Redis Cluster, having
multiple databases would be useless and an unnecessary source of complexity.
Commands operating atomically on a single database would not be possible with
the Redis Cluster design and goals.

Since the currently selected database is a property of the connection, clients
should track the currently selected database and re-select it on reconnection.
While there is no command in order to query the selected database in the current
connection, the `CLIENT LIST` output shows, for each client, the currently
selected database.

@return

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