1.理論就是先彙總在排序

with countzybh as

(select s.zybh,t.zymc,t.備註,count(1) as scount from

student s inner join specialty t on s.zybh=t.zybh

group by s.zybh,t.zymc,t.備註

)

select zybh,zymc,備註, scount ,

rank() over( order by scount desc) as sort

from countzybh

2.直接left join 加case when 即可實現

select s.a , case when t.c is null then s.b else t.c end as c from s left join t on s.c=t.c

如果是空字串就 欄位名= ""

如果是不等於空字元 欄位名 ""

如果是 null值 就是 欄位名 is null 或者 not null