SQL查詢 - 選擇不同的ID與2個額外的列

Question

林在SQL查詢這樣的工作：（由臺訪問的排序）

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

我想選擇不同的火車，和實際的行必須包括第一站和最後一站如：

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

任何人都可以舉手？期待您的光臨！

Answer 1

假設你發現了一些對站定義順序，這樣就可以識別出「最後」和「第一」的一個，下面應該工作：

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 

這假定some_order_column可以用來確定最後和第一站。

它還假定type對於train_id和station的所有組合總是相同的。

顯示的語法是標準的ANSI SQL，應該適用於大多數現代DBMS。