REFERENCE, in turn, is a system type that is the data type of the object identifier column of any typed table.
反过来，REFERENCE是一个系统类型，它是任何类型化表的对象标识符的数据类型。
A definition specifies a variable's type and identifier. A definition may also provide an initial value for the object.
变量定义指定了变量的类型和标识符，也可以为对象提供初始值。
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