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。
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。
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。
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）。
应用推荐