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。

youdao

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。

youdao

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。

youdao