DB2 will be responsible for generating a unique value for the column for each INSERT statement.
对于每条insert语句，DB 2将负责为其中的这一列生成一个惟一的值。
This guarantees that the column values remain unique and frees you from worrying about this column later on (don't try to assign a value to it, though).
这保证列值是惟一的并避免您为这个列操心(但是，不要尝试为它赋值)。
Without this requirement, the itemNumber column isn't guaranteed to be unique because multiple columns could be assigned a NULL value.
如果没有这个要求，itemnumber列就无法保持惟一，因为多个行都可能被分配null值。
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）。
When you attach a unique constraint to a column allowing null values, you ensure that at most one row will have a null value in the constrained column.
当将唯一约束附加到允许空值的列时，请确保在约束的列中最多有一行包含空值。
When you attach a unique constraint to a column allowing null values, you ensure that at most one row will have a null value in the constrained column.
当将唯一约束附加到允许空值的列时，请确保在约束的列中最多有一行包含空值。
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