user_name, ip_address, forwarded_ip_address, client_key, client_key, user_name, client_key, ip_address, and normalized_query_hash correspond to the fields in the system.quotas table.
IPV4_PREFIX_BITS and IPV6_PREFIX_BITS options can only be used when KEYED BY is ip_address or forwarded_ip_address. They correspond to the field in the system.quotas table.
Parameters queries, query_selects, query_inserts, errors, result_rows, result_bytes, read_rows, read_bytes, execution_time, written_bytes, written_rows, failed_sequential_authentications, queries_per_normalized_hash correspond to the fields in the system.quotas_usage table.
ON CLUSTER clause allows creating quotas on a cluster, see Distributed DDL.
Examples
Limit the maximum number of queries for the current user with 123 queries in 15 months constraint:
ALTER
Related: CREATE QUOTA, ALTER, SHOW