约束有两种类型：表级约束和列约束。
Constraints come in two types: table-level constraints and column constraints.
换句话说，模式并不等同于sql列约束。
In other words, schemas are not the equivalent of SQL column constraints.
这些规则提供了更详细的信息，是关于存在一个昵称各个列的关系和约束。
These rules provide more detailed information about the relationships and restrictions that exist on the columns of a nickname.
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