如何从更多列中进行选择，但按1列分组？

Question

SELECT        studentnum
FROM            Atten
WHERE        (att = 'Yes') AND (unitCode = 'MMA1034') 
GROUP BY studentnum
HAVING        (COUNT(*) < 4)

如何选择更多列？例如，student_name？

Answer 1

如果学生信息在“学生”表中，则查询可能如下所示：

SELECT student_name, student_birth_day, studentnum
FROM Student S
RIGHT JOIN (
  SELECT studentnum, count(*) as cnt
  FROM   Attendance
  WHERE (attStatus = 'Yes') 
  AND   (unitCode = 'MMA1034')
  GROUP BY studentnum
  HAVING (COUNT(*) < 4)
) A
ON A.studentnum = S.studentnum