Some time we need to update one table values to another table value in the same database. For that we can use inner join to get the other table column value and update to the current or required table column. In the below example from UserDetails table I have updated class name from ClassDetails tables class name column.
SET ClassName =C.ClassName
FROM UserDetails as U
INNER JOIN ClassDetails as C ON B.ClassID = M.ClassID
WHERE B.TypeID = 'School' AND U.ClassName = ''
The above SQL Query will help you to understand the JOIN or INNER JOIN in update sql query.