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.
这意味着表必须有一个或多个列级或表级约束。
This implies that the table must have one or more columns or table-level constraints.
在这个语法示例中，必须定义一个新列或者定义一个表级约束。
In this syntax example, you must either define a new column or define a new table-level constraint.
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