SQL查询 - 选择不同的ID与2个额外的列

Question

林在SQL查询这样的工作：（由台访问的排序）

TRAIN_ID TYPE STATION 
111  'KC' New York 
111  'KC' Washington 
111  'KC' Boston 
111  'KC' Denver 
222  'FC' London 
222  'FC' Paris 

我想选择不同的火车，和实际的行必须包括第一站和最后一站如：

TRAIN_ID TYPE  FIRSTSTATION LASTSTATION 
111   'KC'  New York  Denver 
222   'FC'  Denver   Paris 

任何人都可以举手？期待您的光临！

Answer 1

假设你发现了一些对站定义顺序，这样就可以识别出“最后”和“第一”的一个，下面应该工作：

WITH numbered_stations AS (
    SELECT train_id, 
     type, 
     row_number() over (partition by train_id order by some_order_column) as rn, 
     count(*) over (partition by train_id) as total_stations 
    FROM the_unknown_table 
) 
SELECT f.train_id, 
     f.type, 
     f.station as first_station, 
     l.station as last_station 
FROM (SELECT train_id, 
     type 
     station 
    FROM numbered_stations 
    WHERE rn = 1 
) f 
    JOIN (SELECT train_id, 
       type, 
       station 
     FROM numbered_stations 
     WHERE rn = total_stations) l 
    ON f.train_id = l.train_id 
ORDER BY train_id 

这假定some_order_column可以用来确定最后和第一站。

它还假定type对于train_id和station的所有组合总是相同的。

显示的语法是标准的ANSI SQL，应该适用于大多数现代DBMS。