谁知道下面这段是什么意思（ABOUT CONSTRAINTS）# Database - 数据库
r*s
1 楼
keys are attributes or sets of attributes that uniquely identify an
object within its class or an entity within its entity set.no two
objects in a class may agree in their values for each of a set of
attributes that constitutes a key.
single-value constraints are requirements that the value in a certain
role be unique,keys are a major source of single-value constraints,since
they require that associated with value(s) for the key attribute(S) are
unique values of the other attributes of the call
object within its class or an entity within its entity set.no two
objects in a class may agree in their values for each of a set of
attributes that constitutes a key.
single-value constraints are requirements that the value in a certain
role be unique,keys are a major source of single-value constraints,since
they require that associated with value(s) for the key attribute(S) are
unique values of the other attributes of the call
