When ON delete set NULL is active, when a record is deleted from the parent table, all records in the child table with matching foreign key values are set to NULL.
当ON DELETE SET NULL规则被激活时，如果一条记录从父表中删除，子表中的所有记录以及匹配的外键值都被设置为NULL。
Drag the parent work items data set to the report, and add an extra column in which to show the child work items.
将上级工作项数据集合拖拉到报表中，并添加一个额外的列以显示下级工作项。
This automatically determines the parent and child relationship between tables, like specifying an exclusive replicate set.
这样便自动决定了这些表之间的父子关系，就像指定了一个专有的复制集。
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