业务规则一般包含一组声明性的语句和约束，然后这些语句和约束可以断言某些操作或目标。
Business rules are usually comprised of sets of declarative statements and constraints that can then predicate certain actions or goals.
尽管s.t .对于声明一个约束来说并不需要在行首，但是这样可以提高代码的可读性。
Although s.t. is not required at the beginning of the line to declare a constraint, it improves the readability of the code.
不要包括可定义声明引用完整性约束的子句。
Do not include any clauses defining declarative referential integrity constraints.
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