itemnumber

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。

An inner join is essentially the intersection of two tables , where the tables are matched by comparing the values of a key column , such as itemNumber .

内连接实际上是两个表的交集，它通过比较关键列（比如itemNumber）的值来匹配表。

On the other hand , if you use the EXISTS operator , all rows are modified because at least one of the itemNumber values exists in the table subquery .

另一方面，如果使用EXISTS操作符，则将修改所有行，因为至少有一个itemNumber值存在于表子查询中。

For example , itemNumber = ( Scalar Subquery ) updates any rows that have a value in the itemNumber column that matches the result of the scalar subquery .

例如，itemNumber=（标量子查询）更新itemNumber列值与标量子查询结果匹配的任何行。

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（NOTNULL）。