Combine the sugar, ketchup, vinegar, and soy sauce in a bowl.
Mix the combined ingredients using a balloon whisk or fork.
Heat a saucepan then pour-in the sugar-ketchup-vinegar-soy sauce mixture. Stir once in a while and bring to a boil.
Pour-in the cornstarch diluted in water then continue stirring until the texture becomes thick.
Turn of the heat and transfer the sweet and sour sauce to a sauce bowl then serve.