In general, DBMSs are multi-user systems, i.e. many users or applications may access the same data in parallel. In order to prevent the interference of user operations, access to the data is controlled by the DBMS through user-specific views and access privileges.
A user-specific view or subschema defines which entities and relationships are visible to the user. The entities and relationships visible to the user must not be identical to those of the global database schema.
Access privileges define what mode of access a user has to the data. Access privileges can be defined for the global database schema, the subschema, or individual entity sets (e.g. tuples in a relation table, objects in a class). The access mode is either read or write. In general, a user can grant his access privileges to other users.