The itemNumber column provides a unique identity for each item (or row) and has an attached column-level constraint that enforces a valid value to always be supplied (NOT NULL).
itemNumber列提供了每个商品（或列）的惟一标识，它上面还有列级约束，强制要求提供正确的值（NOT NULL）。
The itemNumber column provides a unique identity for each item (or row) and has an attached column-level constraint that enforces a valid value to always be supplied (NOT NULL).
itemNumber列提供了每个商品（或列）的惟一标识，它上面还有列级约束，强制要求提供正确的值（NOT NULL）。
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