How to update columns dynamically

There are three things to establish: which column, which row, and which value.

Which column is specified in the SET clause. To designate which column(s) to update, you must name them. If the column is updated conditionally, you must still name it.

Which row is specified in the WHERE clause. This can be based on columns other than those being updated.

Which value is specified, if necessary, using a CASE expression.

update salaries
   set bonus =
       case when job = 'dir' then 1000
            when job = 'mgr' then  500
                             else    0 end
     , salary = salary * 
       case when salary < 45000 then 1.02
                                else 1.04 end
 where location = 'hq'

Bonus for directors and managers, plus a 2% increase for everyone under 45,000 and 4% for everyone over. Applies only to employees at home office.