If a process has already locked a resource, the granting of lock requests by other transactions on the same resource is governed by the lock compatibility matrix within SQL Server. Table 38.5 shows the lock compatibility diagram for the SQL Server Lock Manager, indicating which lock types are compatible and which lock types are incompatible when requested on the same resource.
Table 38.5. SQL Server Lock Compatibility Matrix
For example, if a transaction has acquired a shared lock on a resource, the possible lock types that can be acquired on the resource by other transactions are intent shared, shared, update, and schema stability locks. Intent exclusive, SIX, exclusive, schema modification, and bulk update locks are incompatible with a shared lock and cannot be acquired on the resource until the shared lock is released.