ถ้าผมต้องการ MIN ของ tb_chkpoint.pretest ทำไมใส่ MIN(tb_chkpoint.pretest) ไม่ได้ครับ

select member.name,member.room,tb_chkpoint.posttest,min(tb_chkpoint.pretest) as minper from member,tb_chkpoint where member.member_id=tb_chkpoint.member_id and tb_chkpoint.c_id='1' and tb_chkpoint.u_id='1' and member.room ='isr4b'
group by member.name,member.room,tb_chkpoint.posttest

select 
	member.name,
	member.room,
	min( tb_chkpoint.pretest),
	max( tb_chkpoint.pretest),
	min(tb_chkpoint.posttest), 
	max(tb_chkpoint.posttest) 
from member,tb_chkpoint 
where member.member_id=tb_chkpoint.member_id 
and tb_chkpoint.c_id='1' 
and tb_chkpoint.u_id='1' 
and member.room ='isr4b'