The For Update clause locks the rows of the database table selected by the Select statement. The form is:
FOR UPDATE OF column_expressions
column_expressions is a list of field names in the database table that you intend to update, separated by a comma (,).
The following example returns all rows in the employee database that have a salary field value of more than $20,000. When each record is fetched, it is locked. If the record is updated or deleted, the lock is held until you commit the change. Otherwise, the lock is released when you fetch the next record.
SELECT * FROM emp WHERE salary > 20000 FOR UPDATE OF last_name, first_name, salary
