If there are several rows in the parent table with the same referenced key value, InnoDB performs a foreign key check as if the other parent rows with the same key value do not exist. 如果父表中有几行具有相同的引用键值，InnoDB会执行外键检查，就好像其他具有相同键值的父行不存在一样。For example, if you define a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB does not permit the deletion of any of the parent rows.例如，如果定义了RESTRICT类型约束，并且有一个子行具有多个父行，则InnoDB不允许删除任何父行。

If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the same cascade, it acts like RESTRICT. 如果ON UPDATE CASCADE或ON UPDATE SET NULL重复出现以更新其先前在同一级联期间更新的同一表，则其行为类似于RESTRICT。This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. 这意味着您不能使用自引用的ON UPDATE CASCADE或ON UPDATE SET NULL操作。This is to prevent infinite loops resulting from cascaded updates. 这是为了防止级联更新导致无限循环。A self-referential ON DELETE SET NULL, on the other hand, is possible, as is a self-referential ON DELETE CASCADE. 另一方面，自引用ON DELETE SET NULL是可能的，自引用的ON DELETE CASCADE也是可能的。Cascading operations may not be nested more than 15 levels deep.级联操作的嵌套深度不能超过15层。

In an SQL statement that inserts, deletes, or updates many rows, foreign key constraints (like unique constraints) are checked row-by-row. 在插入、删除或更新许多行的SQL语句中，外键约束（如唯一约束）会逐行检查。When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records that it must examine. 当执行外键检查时，InnoDB会在它必须检查的子记录或父记录上设置共享行级锁。MySQL checks foreign key constraints immediately; the check is not deferred to transaction commit. MySQL会立即检查外键约束；检查不会延迟到事务提交。According to the SQL standard, the default behavior should be deferred checking. 根据SQL标准，默认行为应该是延迟检查。That is, constraints are only checked after the entire SQL statement has been processed. 也就是说，只有在处理完整个SQL语句之后才检查约束。This means that it is not possible to delete a row that refers to itself using a foreign key.这意味着无法删除使用外键引用自身的行。

No storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-integrity constraint definitions. 包括InnoDB在内的任何存储引擎都无法识别或强制执行引用完整性约束定义中使用的MATCH子句。Use of an explicit MATCH clause does not have the specified effect, and it causes ON DELETE and ON UPDATE clauses to be ignored. 使用显式MATCH子句不会产生指定的效果，并且会导致忽略ON DELETE和ON UPDATE子句。Specifying the MATCH should be avoided.应避免指定MATCH。

The MATCH clause in the SQL standard controls how NULL values in a composite (multiple-column) foreign key are handled when comparing to a primary key in the referenced table. SQL标准中的MATCH子句控制在与引用表中的主键进行比较时如何处理复合（多列）外键中的NULL值。MySQL essentially implements the semantics defined by MATCH SIMPLE, which permits a foreign key to be all or partially NULL. MySQL本质上实现了MATCH SIMPLE定义的语义，它允许外键全部或部分为NULL。In that case, a (child table) row containing such a foreign key can be inserted even though it does not match any row in the referenced (parent) table. (It is possible to implement other semantics using triggers.)在这种情况下，可以插入包含此类外键的（子表）行，即使该行与引用的（父）表中的任何行都不匹配。（可以使用触发器实现其他语义。）

MySQL requires that the referenced columns be indexed for performance reasons. However, MySQL does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL.MySQL出于性能原因要求对引用的列进行索引。但是，MySQL并没有强制要求引用的列是UNIQUE或声明为NOT NULL。

A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL but rather an InnoDB extension. 引用非UNIQUE键的FOREIGN KEY约束不是标准SQL，而是InnoDB扩展。The NDB storage engine, on the other hand, requires an explicit unique key (or primary key) on any column referenced as a foreign key.另一方面，NDB存储引擎需要在任何被引用为外键的列上有一个显式的唯一密钥（或主键）。

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. 对于UPDATE或DELETE CASCADE等操作，没有很好地定义对非唯一键或包含NULL值的键的外键引用的处理。You are advised to use foreign keys that reference only UNIQUE (including PRIMARY) and NOT NULL keys.建议您使用仅引用UNIQUE（包括PRIMARY）和NOT NULL键的外键。

MySQL parses but ignores “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL解析但忽略“内联REFERENCES规范”（如SQL标准中定义的），其中引用被定义为列规范的一部分。MySQL accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification. MySQL仅在作为单独的FOREIGN KEY规范的一部分指定时才接受REFERENCES子句。For storage engines that do not support foreign keys (such as MyISAM), MySQL Server parses and ignores foreign key specifications.对于不支持外键的存储引擎（如MyISAM），MySQL Server会解析并忽略外键规范。