ASELECT学号FROM SC WHERE课程号='2001'AND学号IN(SELECT学号FROM SC WHERE课程号='2005’)
BSELECT学号FROM SC WHERE课程号='2001'AND学号=(SELECT学号FROM SC WHERE课程号='2005’)
CSELECT学号FROM SC WHERE课程号='2001'AND课程号='2005'
DSELECT学号FROM SC WHERE课程号='2001'OR'2005'