Question

How do I subtract one table column value from another table column value, and save the value in the first table column in the process?

Thanks for your help.

Answer 1

Where tablename is the table and col1 is the first column and col2 is the second column.

UPDATE tablename SET col1 = (col2 - col1) 

If you are meaning that the values are coming from different tables then the query is much the same:

UPDATE tablename SET col1 = ((SELECT col2 from tablename2) - col1)