constraint属性包含一个布尔表达式，它定义数据必须符合的条件。
The constraint attribute contains a Boolean expression, which defines the condition to which the data is constrained.
constraint属性包含一个布尔表达式，它定义数据必须符合的条件。
The constraint attribute contains a Boolean expression, which defines the condition to which the data is constrained.
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