Since you can set any arbitrary attribute value (and yes, that includes values that you define) to a constraint expression or use it in a constraint, there are a lot of possibilities.

由于可以为约束表达式设置任何任意的属性值(包括您自己定义的值)，或者在约束中使用它，因此存在很多可能。

youdao

The schema constraint count(key('ids', .)) = 1 checks the set of all identifiers to be sure that only one has the current value (in other words whether the identifier is unique).

模式将限定 count(key('ids', .) ) = 1 检查所有的标识符，从而确保只有一个包含当前值（换句话说，确定该标识符是否是惟一的）。

youdao

Now you set the store ID and supplier ID fields so that they do not accept an empty value. Of course, you can add a schema constraint into the XForms model to achieve this.

现在，设置store ID和supplier ID字段以使这两个字段不接受空值。

youdao