2,645. Drinking Doesn't Include Eating
Hilchos Shevuos 4:4
If someone takes an oath not to drink on a certain day, he is allowed to eat because drinking doesn’t include eating. The Ramban opines that in order to be liable one must drink a reviis (about 3.3 ounces), which is the minimum volume of drinking vis-à-vis other prohibitions.
Hilchos Shevuos 4:5
If someone took an oath not to eat on a certain day and he then ate many different types of food, or if he took an oath not to drink and he then drank many different types of beverages, he’s only liable once. Even if he specifies in his oath that he won’t eat meat, bread or beans and he eats all three, he’s only liable once. All of these foods combine to reach the minimum volume of an olive.