Normal functionality is when you highlight something, and then start typing, it will overwrite what you hightlighted. In the code editor, this works fine unless the word you highlighted is a keyword - like WHERE, AND, NULL, etc. If I highlight one of those words, then start typing, it keeps the word and adds what I am typing. I think this should be consistent. If you highlight something, and start typing, it should overwrite it - regardless if it is a keyword or not.