Lock modes: shared, exclusive

Row-level shared locks allow multiple users to read data, but do not allow any users to change that data.

Table-level shared locks allow multiple users to perform read and write operations on the table, but do not allow any users to perform DDL operations.

Multiple users can hold shared locks simultaneously.

An exclusive lock allows only one user/connection to update a particular piece of data (insert, update, and delete). When one user has an exclusive lock on a row or table, no other lock of any type may be placed on it.

Kilde: IBM