UNIQUE CONSTRAINT允許其中的欄位有空值，但是欄位的組合仍必須為唯一
英文說明
WHAT IS THE DIFFERENCE BETWEEN A UNIQUE CONSTRAINT AND A PRIMARY KEY?
| Primary Key | Unique Constraint |
|---|---|
| None of the fields that are part of the primary key can contain a null value. | Some of the fields that are part of the unique constraint can contain null values as long as the combination of values is unique. |
Oracle does not permit you to create both a primary key and unique constraint with the same columns.
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