MySQL可以通过CREATE TABLE 或者ALTER TABLE 来定义MySQL外键。下面就为您介绍这两种方法相关的语法，希望对您学习MySQL外键方面有所帮助。
CREATE TABLE 定义MySQL外键的语法：
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
[(create_definition,…)]
create_definition:
column_definition
| [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,…) [reference_definition]
column_definition:
col_name type [NOT NULL | NULL] [DEFAULT default_value]
[AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY]
[COMMENT ’string’] [reference_definition]
index_col_name:
col_name [(length)] [ASC | DESC]
reference_definition:
REFERENCES tbl_name [(index_col_name,…)]
[MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]
[ON DELETE reference_option]
[ON UPDATE reference_option]
reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION
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ALTER TABLE 定义MySQL外键的语法：
ALTER [IGNORE] TABLE tbl_name
alter_specification [, alter_specification] …
alter_specification:
| ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,…)
| ADD [CONSTRAINT [symbol]] UNIQUE [INDEX] [index_name] [index_type] (index_col_name,…)
| ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,…) [reference_definition]
| DROP FOREIGN KEY fk_symbol
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