sql - 如何使 SQL JOIN 仅返回具有最新状态代码的一行？

Question

以下 SQL 语句:

SELECT f.id, 
       f.front, 
       f.back, 
       h.statusChangedIdCode, 
       h.whenCreated,
       h.historyIdCode 
FROM dpod_site_flashcards AS f 
   JOIN dpod_site_historyItems AS h ON f.id = h.itemId 
WHERE f.id = 216 
  AND historyIdCode = 'statusChanged' 
ORDER BY h.whenCreated

给我以下输出。 如何更改它以使其仅返回具有最新 statusChangedIdCode 的一行？

Answer 1

SELECT f.id, 
       f.front, 
       f.back, 
       h.statusChangedIdCode, 
       h.whenCreated,
       h.historyIdCode 
FROM dpod_site_flashcards AS f 
   JOIN dpod_site_historyItems AS h ON f.id = h.itemId 
WHERE f.id = 216 
  AND h.historyIdCode = 'statusChanged' 
  AND h.whenCreated = (select max(whenCreated)
                       from dpod_site_historyItems h2
                       where h2.itemId = h.itemId
                         and h2.historyIdCode = 'statusChanged')
ORDER BY h.whenCreated

或者，如果您有支持窗口功能的现代 DBMS:

SELECT f.id, 
       f.front, 
       f.back, 
       h.statusChangedIdCode, 
       h.whenCreated,
       h.historyIdCode 
FROM dpod_site_flashcards AS f 
   JOIN (
     select itemid, 
            historyIdCode,
            whenCreated,
            statusChangedIdCode,
            row_number() over (partition by itemid order by whenCreated desc) as rn 
     from dpod_site_historyItems 
     where historyIdCode = 'statusChanged' 
   ) h ON f.id = h.itemId AND h.rn = 1
WHERE f.id = 216 
ORDER BY h.whenCreated

当您为多个 dpod_site_flashcards.id 选择行时，这两种情况也适用LIMIT 解决方案不会这样做。