这些SQL语句怎么写？？？？！！！100高分！！！！！！

Question

SELECT [T1].课程号, IIf(ISNULL([T2].先行课程号),"--",[T2].先行课程号) AS 先行课程号
FROM TableB AS T1 LEFT JOIN TableB AS T2 ON [T1].先行课程号=[T2].课程号;

Answer 1

select 课程表.pc# from 课程表,{select pc# from 课程表 where c#=?} as Course where 课程表.c#=course.pc#

Answer 2

(已验证！)
Students(StudentID, Name)
         1          1
         2          2
         3          3
         4          4
         5          5
Subjects(SubjectID, Name)
         C1         C1
         C2         C2
         C3         C3
         C4         C4
         C5         C5
Grades(StudentID, SubjectID, Grade)
         1          C1         A
         1          C2         A
         1          C3         A
         1          C5         B
         2          C1         B
         2          C2         C
         2          C4         C
         3          C2         B
         3          C3         C
         3          C4         B
         4          C3         B
         4          C4         D
         5          C1         D
         5          C2         D
         5          C3         D
         5          C4         D
         5          C5         DAnswer to questoin one  SELECT Students.*
  FROM Students
  WHERE NOT EXITS(
    SELECT *
    FROM Grades
    WHERE EXITS(
      SELECT *
      FROM Subjects
      WHERE  Grades.StudentID   = Students.StudentID AND
             Subjects.SubjectID = Grades.SubjectID AND
             Subjects.SubjectID = 'C3'))Answer to questoin twoSELECT Students.*
FROM Students
WHERE NOT EXISTS(
  SELECT *
  FROM Subjects
  WHERE NOT EXISTS(
    SELECT *
    FROM Grades
    WHERE Subjects.SubjectID = Grades.SubjectID AND
           Students.StudentID = Grades.StudentID))

Answer 3

因为嵌套的先行课层数不确定，难以用一个SQL给出。
最简单的方法是循环：
dim strC# as string
do 
set rs = cn.execute("select cn, [pc#] from kb where [c#]='" & _
    strC#  & "'")
if not isnull(rs!pc#) then strC# = rs!pc#
loop until isnull(rs!pc#)