選擇列，它們各具地方條件SQL

Question

我有一個表包含了所有的名與不同的類，像這樣：

Name Class 

Jack A 

Nick B 

Simon C 

David B 

Linda B 

Alice C 

現在我希望得到一個表格，以A級，B，C作爲列，其中包含其尊重類內的名稱：

A----------B----------C 

Jack------Nick-------Simon 

----------David------Alice 

----------Linda----------- 

如何在SQL查詢中獲得此類表？對不起格式不好，不知道如何在SO中創建表格。

Answer 1

這是一種使用聚合的方法。這可能是最簡單的方法：

select max(case when class = 'a' then name end) as a, 
     max(case when class = 'b' then name end) as b, 
     max(case when class = 'c' then name end) as c 
from (select name, class, row_number() over (partition by class order by (select NULL)) as seqnum 
     from nameclasses 
    ) nc 
group by seqnum 
order by seqnum; 

這是我張貼的原始方法。它不使用聚合，但它確實有很多的連接：

select a.name as a, b.name as b, c.name as c 
from (select name, row_number() over (order by (select NULL)) as seqnum 
     from nameclasses nc 
     where class = 'A' 
    ) a full outer join 
    (select name, row_number() over (order by (select NULL)) as seqnum 
     from nameclasses nc 
     where class = 'B' 
    ) b 
    on a.seqnum = b.seqnum full outer join 
    (select name, row_number() over (order by (select NULL)) as seqnum 
     from nameclasses nc 
     where class = 'c' 
    ) c 
    on c.seqnum = coalesce(a.seqnum, b.seqnum) 
order by coalesce(a.seqnum, b.seqnum, c.seqnum); 

Answer 2

請求結束時使用「group by」功能，如下所示：「通過a.tata從組中選擇a.toto，a.tata」。 http://www.w3schools.com/sql/sql_groupby.asp

Answer 3

你問的數據是不是真的很關係，在這種情況下，你通常會從做取得更好的成績在客戶端應用程序中工作。但是，這往往是不可能的：

SELECT A.Name as A, B.Name as B, C.Name AS C 
FROM 
    (select Name, row_number over (order by name) as ordinal from table where class = 'A') A 
FULL JOIN 
    (select Name, row_number over (order by name) as ordinal from table where class = 'B') B 
    ON B.ordinal = A.ordinal 
FULL JOIN 
    (select Name, row_number over (order by name) as ordinal from table where class = 'C') C 
    ON C.ordinal = coalesce(A.ordinal, B.ordinal)