sql - 有重复键时选择指定值

Question

所以我有一张桌子，例如租来的物品

 id | itemID | state
 3  |      2 |     0
 2  |      2 |     1
 1  |      2 |     1

状态表示该项目是否在过去租用。0 = currently rented

包含项目的表：

id | item   | condition
1  | showel |      good
2  |    saw |        ok

现在我想选择当前未租用的物品，我最终得到了这个：

SELECT * FROM
(
  SELECT CASE WHEN b.itemID = a.id AND b.state = 0 THEN 1
  ELSE 0 END AS itemState, a.condition AS condition
  FROM items AS a
  LEFT JOIN rented AS b ON a.id = b.item_id
) AS states
WHERE states.itemState = 0

但是 SQL 返回的是 saw[id=2] 当前是免费的，但事实并非如此。

有什么办法可以修复 SQL 语句？

// 编辑：我需要在租用的表中保留重复的 itemID

Answer 1

如果我正确理解你所追求的，你可以这样实现：

SELECT outerA.*
  FROM items AS outerA
    LEFT JOIN (SELECT innerA.*
                 FROM rented AS innerA
                   JOIN (SELECT MAX(id) AS id 
                           FROM rented 
                           GROUP BY itemID) AS innerB
                     ON innerB.id = innerA.id) AS outerB
      ON outerB.itemID = outerA.id
  WHERE outerB.state IS NULL
    OR outerB.state = 1;

它将选择从未租用过的任何项目，或者它在rented表中的最后一个条目表明它当前没有租用。

另请参阅这个简短的演示。