As stated it does for InnoDB. At first I thought it was strange that many other (in particular MS SQL and DB2) doesn't. TableSpace scans are only better than index scans when there are very few table rows - so for the vast majority of cases a foreign key would want to be indexed. Then it kind of hit me - this doesn't necessarily mean it has to be a stand alone (one column) index - where it is in MySQL's automatic FK Index. So, may be that is the reason MS SQL, DB2 (Oracle I'm not sure on) etc leave it up to the DBA; after all multiple indexes on large tables can cause issues with performance and space.

如果你做ALTER TABLE(而不是CREATE TABLE)，你不会自动得到索引，至少根据文档(链接是5.1的，但5.5也是一样的):

[…当您使用ALTER table向表添加外键约束时，请记住首先创建所需的索引。

As stated it does for InnoDB. At first I thought it was strange that many other (in particular MS SQL and DB2) doesn't. TableSpace scans are only better than index scans when there are very few table rows - so for the vast majority of cases a foreign key would want to be indexed. Then it kind of hit me - this doesn't necessarily mean it has to be a stand alone (one column) index - where it is in MySQL's automatic FK Index. So, may be that is the reason MS SQL, DB2 (Oracle I'm not sure on) etc leave it up to the DBA; after all multiple indexes on large tables can cause issues with performance and space.

是的，Innodb提供了这个。你可以在foreign key子句后面加上一个外键名称，或者让MySQL为你创建一个名称。MySQL自动创建一个名为foreign_key_name的索引。

CONSTRAINT constraint_name
FOREIGN KEY foreign_key_name (columns)
REFERENCES parent_table(columns)
ON DELETE action
ON UPDATE action

这是不可能得到索引键自动使用

ALTER TABLE (NAME OF THE TABLE) ADD INDEX (FOREIGN KEY)

你创建的表的名称，例如photos和外键，例如照相_id。代码应该是这样的

ALTER TABLE photographs ADD INDEX (photograph_id);

是的，但只在innodb上。Innodb是目前唯一实现了外键的表格式。