1. A database system comprising: a plurality of databases, at least two of the plurality of databases are stored with one or more data collections composed of tables with the same structure, a federated view of the data collection is created, by at least one computer processor, on each of the at least two databases; and a request routing layer for routing, in response to a query request crossing the data collections, the query request crossing the data collections to one of the at least two databases according to a predetermined routing rule, so as to query by using the federated view of the database; wherein the predetermined routing rule comprises: a first routing rule for routing a cross-tenant data query request from a tenant to the database in which tenant data of the tenant is located, a second routing rule for routing a cross-tenant data query request from a tenant to the database in which most of tenant data involved is located and a third routing rule for routing a cross-tenant data query request to the database with the lowest load based on the status of the databases.