When a transaction sets a shared lock on a database object, then other transactions can continue to gain read access to the database object, and other shared locks can be set for this database object. However, you cannot set an exclusive lock, or change the database object.
Shared locks can be set for the database catalog, tables, or a table row.
A shared lock is released implicitly at the end of the transaction by the database system or it is released explicitly by an UNLOCK Statement executed by the user.
A shared or read lock is also known as a SHARE lock.