如果从数据库返回了一行，则使用非空的用户名和密码成员变量来填充userbean。
If a row is returned from the database, the UserBean is populated with non-null user name and password member variables.
只有在username或password之一非空的情况下，代码才会检验登录数据或颁发visa。
Only if one of either username or password is populated the code will proceed to verify the logon data or even issue a visa.
这种校验会通过拒绝空值，强迫加上非空[notNULL]字段的约束，但是也只有在这种情况下，你的数据库会拒绝空值。
This new validation will enforce NOT null column constraints by rejecting null values, but only in situations where your database would reject a null value.
对于数据库中包含具有非空值的名称和金额字段的每个文档来说，将在视图中创建一个结果行。
For each document in the database that has name and amount fields with non-null values, a result row is created in the view.
一些约束不会有变化（非空、数据库中的最大长度），但有一些会因国家而不同。
Some constraints will not change (not null, maximum length in the database), but some will vary with the country.
设计域完整性属性。考虑CHECK约束，数据类型和是否非空。
Design attribute domain integrity. Considerations include CHECK constraints, data types, and nullability.
设计域完整性属性。考虑CHECK约束，数据类型和是否非空。
Design attribute domain integrity. Considerations include CHECK constraints, data types, and nullability.
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