改下:
select * from tb_student a,tb_score b
where (a.student_no*= b.student_no)
等价于:
select * from tb_student a,tb_score b
where (b.student_no is null) or (a.student_no= b.student_no)
select * from tb_student a,tb_score b
where (a.student_no*= b.student_no)
等价于:
select * from tb_student a,tb_score b
where (a.student_no is null) or (a.student_no= b.student_no)