外键是否总是引用另一个表中的唯一键? [英] does foreign key always reference to a unique key in another table?

问题描述

子表中的外键(单列)是否不可能引用具有某些重复值的父键?

Is it not possible that foreign key(single column) in a child table references to a parent key which has some duplicate values?

推荐答案

通过SQL标准，外键必须引用父表的主键或唯一键.如果主键具有多个列，则外键必须具有相同的列数和顺序.因此，外键引用父表中的唯一行；不能重复.

By the SQL standard, a foreign key must reference either the primary key or a unique key of the parent table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore the foreign key references a unique row in the parent table; there can be no duplicates.

发表您的评论

如果T.A是主键，则不能，您不能有任何重复项.任何主键都必须是唯一的并且不能为空.因此，如果子表具有引用父级主键的外键，则它必须与一个非空的唯一值匹配，因此在父表中恰好引用了一行.在这种情况下，您不能创建引用多个父行的子行.

If T.A is a primary key, then no you can't have any duplicates. Any primary key must be unique and non-null. Therefore if the child table has a foreign key referencing the parent's primary key, it must match a non-null, unique value, and therefore references exactly one row in the parent table. In this case you can't make a child row that references multiple parent rows.

您可以创建一个子行，其外键列为NULL，在这种情况下，它不引用父表中的任何行.

You can create a child row whose foreign key column is NULL, in which case it references no row in the parent table.

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