在mysql修改表为外键的示例：

country 表是父表，country_id是主键，city是子表，外键为country_id，和country表的主键country_id对应，在创建表的时候添加外键，示例：

createtablecountry(
country_idsmallintunsignednotnullauto_increment,
countryvarchar(50)notnull,
last_updatetimestampnotnulldefaultcurrent_timestamponupdatecurrent_timestamp,
primarykey(country_id)
)engine=INNODBdefaultcharset=utf8;
CREATETABLE`city`(
`city_id`smallint(5)unsignedNOTNULLauto_increment,
`city`varchar(50)NOTNULL,
`country_id`smallint(5)unsignedNOTNULL,
`last_update`timestampNOTNULLdefaultCURRENT_TIMESTAMPonupdateCURRENT_TIMESTAMP,
PRIMARYKEY(`city_id`),
KEY`idx_fk_country_id`(`country_id`),
CONSTRAINT`fk_city_country`FOREIGNKEY(`country_id`)REFERENCES`country`(`country_id`)ondeleterestrictONUPDATECASCADE
)ENGINE=InnoDBDEFAULTCHARSET=utf8;

在建表后添加外键的示例：

ALTERTABLEcityADDFOREIGNKEY(country_id)REFERENCES`country`(country_id);