student(sno,sname,sage,ssex,sdept) course(cno,cname,credit,ctime) sc(sno,cno,grade) 1．建立学生表的insert触发器，若向学生表中插入一条记录，则自动向选课(sc)表中插入一行，令该生选一门选修人数最多的课。写sql语句

以下是向学生表中插入记录时自动向选课表中插入一行的 SQL 语句：

CREATE TRIGGER insert_sc AFTER INSERT ON student
FOR EACH ROW
BEGIN
  INSERT INTO sc (sno, cno, grade)
  SELECT NEW.sno, cno, NULL
  FROM (
    SELECT cno, COUNT(*) AS num
    FROM sc
    GROUP BY cno
    ORDER BY num DESC
    LIMIT 1
  ) AS t
  WHERE cno NOT IN (
    SELECT cno
    FROM sc
    WHERE sno = NEW.sno
  );
  UPDATE course
  SET num = num + 1
  WHERE cno = (
    SELECT cno
    FROM (
      SELECT cno, COUNT(*) AS num
      FROM sc
      GROUP BY cno
      ORDER BY num DESC
      LIMIT 1
    ) AS t
  );
END;

该触发器会在向学生表中插入记录时自动执行，向选课表中插入一行数据，使该学生选修人数最多的尚未选修的课程。同时，更新选修课程的选修人数。