具有不同属性的实体取决于其类型？
问题描述：
试图根据给定的大学项目简介设计ER图。具有不同属性的实体取决于其类型？
我很困惑，我应该如何处理这个问题：
The items sold in the Food Truck can be of different types: burritos and
beverages. Every item have an ID, a description and a price. Assume that every
Food Truck has infinite stock of each item (i.e. we do not need to track stock levels
in each Food Truck).
All Burritos come with rice, a type of bean, a filling, and a set of optional
toppings. Burritos are priced by size (Mini, Regular, and Grande). Bean types will
vary. Chipp will start by offering two types: black beans and red beans. Burrito
fillings will vary (depending on the season). There are at least 3 types of Burrito
fillings and there should be a vegetarian option.
A Burrito may optionally have toppings: lettuce, tomato, and mild and hot
salsa. Toppings are free, but Chipp will also offer guacamole as a topping for which
there is an extra charge.
The Food Truck also sells different types of refreshing beverages, both
alcoholic and non-alcoholic. All beverages have a size measured in milliliters (just in
case Chipp takes his Food Truck business over the Channel to mainland Europe).
我有这么远的解决方案是通过使两个弱实体，两者有关系，是这样的：
这是处理问题的正确方法吗？
答
陈的原始符号没有子类型的符号。没有弱密钥的弱实体产生相同的结果。您的方法在该框架内是正确的。然而，在相同的原始记号中，弱实体集与识别关系（双边钻石）相关联，并且在实体集合和关系之间用双线表示总体参与，而不是基本指标（min，max）样式。这不是坚持原始记法的建议，但在这些方面针对您的教科书验证您的答案可能是一个好主意。
已经开发了许多不同的扩展符号来表示子类型，并指出不相关性，原始符号不能。如果你的课程中包含这些内容，我建议你使用它们，因为它们更具表现力。
请注意鳄梨酱的额外收费要求，您的图表还没有包括在内。最后，您将价格表示为项目的派生属性，但我没有看到可以从中计算出的其他属性。