HeidiSQL is a great tool to manage MySQL and other supported databases. One useful feature it has is to remember last commands used. I love this feature, however, each time I moved on to another project, the previous command will be there. It has never bothered me but somehow, I just have to know how to clear it. So I began to search and found from here that it was easy.
Solution: To reset the default Query tab to blank, just double click on the default Query tab.