产品

CREATE TABLE if NOT EXISTS `PRODUCTS` (
   `ID` INT unsigned NOT NULL AUTO_INCREMENT,
   `COMPANY_ID` INT(10) unsigned NOT NULL,
   `PRODUCT_CODE` VARCHAR(5) NOT NULL,
   `PRODUCT_NAME` VARCHAR(15) NOT NULL,
   PRIMARY KEY (`ID`),
   UNIQUE INDEX UNIQUE_COMAPNY_UNIT_CODE (`COMPANY_ID`, `PRODUCT_CODE`)
)

服务

ALTER TABLE services
ADD COLUMN `PRODUCT_CODE` VARCHAR(5),
ADD FOREIGN KEY (`PRODUCT_CODE`) REFERENCES PRODUCTS(`PRODUCT_CODE`) ON DELETE CASCADE ON UPDATE CASCADE;

但是当我尝试创建上外键时，它总是给我这个错误 missing index in the references table products ... 如何在服务表中生成产品代码参考产品表中的产品代码？