Max和鮮明的SQL查詢

Question

set search_path = 'project'; 

select g.firstname,g.lastname, g.rating, sh.showname 
from guest g, shows sh, timeslot ts 
where sh.shownumber = ts.shownumber 
and ts.guestnumber = g.guestnumber 
order by sh.showname 

這是結果

"Charlie ";"Sheen";7.2;"Cooking Show" 
"Charlie";"Sheen";7.2;"Cooking Show" 
"Carl";"Makky";7.0;"Cooking Show" 
"Barack";"Obama";6.7;"Fitness Mania" 
"Vladimir";"Putin";8.2;"Fitness Mania" 
"kim";"jung";5.3;"Fitness Mania" 
"kim";"jung";5.3;"Gamers Heaven" 
"Justin";"Trudeau";8.5;"Kids Play Time" 
"Charlie";"Sheen";7.2;"Kids Play Time" 
"ellen";"page";9.2;"Weather News" 

我要的是收視率最高的客人每場，所以它應該有5條記錄，其中最高等級的每個可用顯示一個記錄客人

Answer 1

如果你需要每場只有一個客人 - 您可以使用DISTINCT ON：

select distinct on (sh.showname) g.firstname, g.lastname, g.rating, sh.showname 
from guest g, shows sh, timeslot ts 
where sh.shownumber = ts.shownumber 
and ts.guestnumber = g.guestnumber 
order by sh.showname, g.rating desc 

Answer 2

這是row_number()功能良好的應用：

select firstname, lastname, rating, showname 
from (select g.firstname, g.lastname, g.rating, sh.showname, 
      row_number() over (partition by sh.showname order by g.rating desc) as seqnum 
     from guest g shows sh join 
      timeslot ts 
      on sh.shownumber = ts.shownumber join 
      guest g 
      on ts.guestnumber = g.guestnumber 
    ) gr 
where seqnum = 1 
order by showname; 

我還修改代碼使用明確的join語法。

如果您想要每場演出五位嘉賓，請使用seqnum <= 5。