blocks|key|483411|text|外键和主键有时是相关的，但并不总是相关的。表中的外键字面意思是“这个字段中的任何值都必须存在于另一个表中-->这里”。这个值在另一个表中是否是PK是无关紧要的--它必须存在，而且它必须是唯一的。这可能符合作为主键的要求，但不是必需的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|483412|您可以使用复合外键，例如，在(愚蠢的)地址簿中，您列出一个人的电话号码的子表可以使用(firstname,+lastname)键，但这会遇到"ok，John+Smith是哪个号码的问题吗？“。|offset|length|style|CODE|483413|entityMap^0|0|16|L|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

foreign keys and primary keys are SOMETIMES related, but not always. A foreign key in a table literally just means "whatever value is in this field MUST exist in this other table over ---> here". Whether that value is a PK or not in that other table is irrelevant - it just has to exist, and it has to be unique. That may fit the bill of being a primary key, but being primary is NOT required.

You can have composite foreign keys, e.g. in a (silly) address book, the child table you list a person's phone numbers in COULD be keyed with <code>(firstname, lastname)</code>, but that runs into the problem of "ok, which John Smith does this number belong to?".

blocks|key|483430|text|在大多数数据库中，外键引用必须是对主键或唯一键的引用(并且允许使用NULL)。MySQL建议这样做，但不需要它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|483431|但是，系统不强制要求引用的列是唯一的或声明为NULL。对非唯一键或包含空值的键的外键引用的处理对于诸如UPDATE或DELETE级联之类的操作没有很好的定义。建议您使用只引用唯一(包括主键)和非空键的外键。|blockquote|483432|是否需要引用外键关系的主键？首先，您甚至不需要声明外键关系。我认为它们很重要，因为它们允许数据库维护引用的完整性。但是，它们并不是必需的。基于外键的存在，查询中也没有任何语义差异(例如，NATURAL+JOIN不使用它们)。优化可以使用声明的关系。|483433|其次，如果要声明外键，我建议使用引用表的主键。这毕竟是拥有主键的主要原因之一。|483434|entityMap|0|LINK|mutability|MUTABLE|url|https://dev.mysql.com/doc/refman/5.7/en/create-table-foreign-keys.html^0|X|4|13|H|0|0|0|2L|C|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$]]|$1|I|3|J|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|K|3|L|5|6|7|14|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

In most databases, foreign key references must be to primary keys or unique keys (and <code>NULL</code>s are allowed). <a href="https://dev.mysql.com/doc/refman/5.7/en/create-table-foreign-keys.html" rel="nofollow">MySQL recommends this but does not require it</a>:

<blockquote>
 However, the system does not enforce a
 requirement that the referenced columns be UNIQUE or be declared NOT
 NULL. The handling of foreign key references to nonunique keys or keys
 that contain NULL values is not well defined for operations such as
 UPDATE or DELETE CASCADE. You are advised to use foreign keys that
 reference only UNIQUE (including PRIMARY) and NOT NULL keys.
</blockquote>

Do you need to reference primary keys for a foreign key relationship? First, you don't even need to declare foreign key relationships. I think they are important because they allow the database to maintain referential integrity. But, they are not required. Nor is there any semantic difference in queries, based on the presence of foreign keys (for instance, <code>NATURAL JOIN</code> does not use them). The optimize can make use of the declared relationship.

Second, if you are going to declare the foreign key, I would recommend using the primary key of the referenced table. That is, after all, one of the major reasons for having primary keys.

blocks|key|675529|text|一般来说，你通常不会像这样双向地做外键。如果这样做，这意味着这两个表存在于1到1的关系中:每个taco都有一个类型，每个taco类型只能由一个taco使用。如果您有这样的关系，那么没有任何理由将它们放在单独的表中，它们可能只是同一表中的附加列。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|675530|通常外键用于一对多或多对多的关系.一个一对多的关系将是如果许多不同的玉米饼可以是相同的类型。他们每人都有Taco_Type_ID外键。|offset|length|style|CODE|675531|对于多到多的关系，通常使用单独的关系表。|675532|CREATE+TABLE+Taco_Types+(
++++Taco_ID+INT,+--+FK+to+Table1.Taco_ID
++++Taco_Type_ID+INT,+--+FK+to+Table2.Taco_Type_ID
++++PRIMARY+KEY+(Taco_ID,+Taco_Type_ID)
);|code-block|syntax|javascript|675533|entityMap^0|0|1G|C|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|U|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|V|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

In general, you don't usually make foreign keys bidirectionally like that. If you do, it means that the two tables exist in a 1-to-1 relationship: Each taco has a type, and each taco type can only be used by one taco. If you have a relationship like this, there's not really any reason to have them in separate tables, they could just be additional columns in the same table.

Normally foreign keys are used for 1-to-many or many-to-many relationships.
A 1-to-many relationship would be if many different tacos can be of the same type. They each have <code>Taco_Type_ID</code> foreign key.

For a many-to-many relationship, you typically use a separate relation table.

<pre><code>CREATE TABLE Taco_Types (
 Taco_ID INT, -- FK to Table1.Taco_ID
 Taco_Type_ID INT, -- FK to Table2.Taco_Type_ID
 PRIMARY KEY (Taco_ID, Taco_Type_ID)
);
</code></pre>

This may sound confusing and or simple but..

If I use a foreign key from Table B in Table A, which has a separate primary key. Do I need to include Table A's primary key as a foreign key in Table B?

Thanks!

========================================================================

EDIT:

Okay, let me try and clarify my question a bit. 

<a href="https://i.stack.imgur.com/lrw3Y.png" rel="nofollow"><img src="https://i.stack.imgur.com/lrw3Y.png" alt="EXAMPLE"></a>

In the case above, should I use Taco_ID as a FK in Table 2? Or is does it completely unnecessary?

Use of Primary Key as Foreign Key in Foreign Key Table

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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这听起来可能令人困惑或简单，但是..。如果我使用表A表B中的外键，表A中有一个单独的主键。I是否需要将表A的主键作为外键包含在表B中？谢谢!========================================================================编辑：好吧，让我试着澄清一下我的问题...

问外键表中主键作为外键的使用
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