多列外键：将单个列设置为Null“on Delete”而不是全部

Question

example/SQL：下面的SQL说明了这个示例，但是最后一个语句将不起作用，因为最后一个外键要求lectureId和groupId都是可空的，但是使这两个都为可空意味着删除组也将lectureId设置为空。

CREATE TABLE lectures (
  lectureId INT NOT NULL,
  title VARCHAR(10) NOT NULL,
  PRIMARY KEY (lectureId)
 );

CREATE TABLE groups (
  lectureId INT NOT NULL,
  groupNo INT NOT NULL,
  title VARCHAR(10) NOT NULL,
  PRIMARY KEY (lectureId,groupNo),
  FOREIGN KEY (lectureId) REFERENCES lectures (lectureId)
    ON UPDATE CASCADE ON DELETE CASCADE
 );

CREATE TABLE studentListed (
  studentId INT NOT NULL,
  lectureId INT NOT NULL,
  groupNo INT NULL,
  PRIMARY KEY (studentId,lectureId),
  FOREIGN KEY (lectureId) REFERENCES lectures (lectureId)
    ON UPDATE CASCADE ON DELETE CASCADE,
  FOREIGN KEY (lectureId,groupNo) REFERENCES groups (lectureId,groupNo)
    ON UPDATE CASCADE ON DELETE SET NULL
 );

Answer 1

经过一些研究，似乎不能使用外键来实现这个特定的需求。

最好的解决方案似乎是混合使用外键和触发器。

对于给定的示例，问题可以通过以下语句解决：

CREATE TABLE lectures (
  lectureId INT NOT NULL,
  title VARCHAR(10) NOT NULL,
  PRIMARY KEY (lectureId)
 );

CREATE TABLE groups (
  lectureId INT NOT NULL,
  groupNo INT NOT NULL,
  title VARCHAR(10) NOT NULL,
  PRIMARY KEY (lectureId,groupNo),
  FOREIGN KEY (lectureId) REFERENCES lectures (lectureId)
    ON UPDATE CASCADE ON DELETE CASCADE
 );

CREATE TABLE studentListed (
  studentId INT NOT NULL,
  lectureId INT NOT NULL,
  groupNo INT NULL,
  PRIMARY KEY (studentId,lectureId),
  FOREIGN KEY (lectureId) REFERENCES lectures (lectureId) 
    ON UPDATE CASCADE ON DELETE CASCADE,
  FOREIGN KEY (lectureId,groupNo) REFERENCES groups (lectureId,groupNo)
    ON UPDATE CASCADE ON DELETE CASCADE
 );

CREATE TRIGGER GroupDelete BEFORE DELETE ON groups
FOR EACH ROW
  UPDATE studentListed SET studentListed.groupNo = NULL
    WHERE studentListed.lectureId = OLD.lectureId
    AND studentListed.groupNo = OLD.groupNo;