MySQL设置外键

CREATE TABLE SPJ(
  SNO CHAR(2) NOT NULL ,
  PNO CHAR(2) NOT NULL ,
  JNO CHAR(4) NOT NULL ,
  QTY INT NOT NULL ,
  PRIMARY KEY (SNO,PNO,JNO,QTY)
);

# 设置外键
ALTER TABLE SPJ ADD CONSTRAINT fk_SPJ_SS
FOREIGN KEY (SNO) REFERENCES SS (SNO);

ALTER TABLE SPJ ADD CONSTRAINT fk_SPJ_P
FOREIGN KEY (PNO) REFERENCES P (PNO);

ALTER TABLE SPJ ADD CONSTRAINT fk_SPJ_J
FOREIGN KEY (JNO) REFERENCES J (JNO);