Student(sno,sname,ssex,sage,clno); Course(cno,cname,credit); Class(clno,speciality,inyear,number,monitor); Grade(sno,cno,gmark); 查询选3学分的课最多的学生的学号、姓名和课程数

可以使用以下 SQL 语句来实现：

SELECT s.sno, s.sname, COUNT(*) AS course_count
FROM Student s, Course c, Grade g
WHERE s.sno = g.sno AND c.cno = g.cno AND c.credit = 3
GROUP BY s.sno, s.sname
HAVING COUNT(*) = (
  SELECT MAX(count_)
  FROM (
    SELECT COUNT(*) AS count_
    FROM Student s, Course c, Grade g
    WHERE s.sno = g.sno AND c.cno = g.cno AND c.credit = 3
    GROUP BY s.sno
  ) t
)

该语句通过连接三个表 Student、Course 和 Grade，筛选出选课学分为 3 的成绩记录，再按学号和姓名分组统计每位学生选了多少门 3 学分的课程。最后，通过子查询找出选课门数最多的学生，并返回其学号、姓名和课程数。