How to get min/max of two integers in Postgres/SQL?

You want the inline sql case:

set my_column = case when my_column - 10 > 0 then my_column - 10 else 0 end

max() is an aggregate function and gets the maximum of a row of a result set.

Edit: oops, didn't know about greatest and least in postgres. Use that instead.

Have a look at GREATEST and LEAST.

UPDATE my_table
SET my_column = GREATEST(my_column - 10, 0);