選擇基於沒有組一列獨特的結果通過

Question

我有如下數據（小提琴 - http://sqlfiddle.com/#!6/9eecb7db59d16c80417c72d1/3757）：

SELECT * FROM 
(
    SELECT 1 AS Id1, 1000 AS Id2, NULL AS Item1, 1 AS Item2 --this should be removed from my results 
    UNION SELECT 1 AS Id1, 1000 AS Id2, NULL AS Item1, 2 AS Item2 
    UNION SELECT 3 AS Id1, 1000 AS Id2, 0 AS Item1, 0 AS Item2 
    UNION SELECT 4 AS Id1, 1000 AS Id2, 1 AS Item1, 1 AS Item2 

    --BELOW DATA DIFFERS FROM THE ABOVE ONLY ON Id2 
    UNION SELECT 1 AS Id1, 2000 AS Id2, NULL AS Item1, 1 AS Item2 --this should be removed from my results 
    UNION SELECT 1 AS Id1, 2000 AS Id2, NULL AS Item1, 2 AS Item2 
    UNION SELECT 3 AS Id1, 2000 AS Id2, 0 AS Item1, 0 AS Item2 
    UNION SELECT 4 AS Id1, 2000 AS Id2, 1 AS Item1, 1 AS Item2 
) AS TestData 

我需要過濾TestData，只得到不同Id1, Id2。結果必須符合以下規則：

1. 下面所有的規則都必須遵循Id2分組（我的意思是下面僅適用於由Id2列定義行組）。這意味着TestData數據集包含兩組 - 1000,2000
2. 所有行Item1比賽Item2是好的，應該是始終返回
3. 如果2.不被滿足，則取該行只有Item1是NULL和Item2沒有匹配的任何地方TestData（規則1.仍然適用 - 所以，我們需要通過當前組數據集中查找）

總而言之，我需要從TestData得到每個組的第一行旁邊的所有Id。另外，請注意，我知道我可以簡單地致電SELECT DISTINCT並擁有適當的Id，但在此情況並非如此。由於Item2已在組中的匹配項（規則3），所以我的問題比上面描述的更大，基本上我想知道的是如何從上述數據中的每個組中刪除第一行。

Answer 1

這將返回您所期望的行。 COALESCE是一個快捷方式，所以如果您的數據中可能有負值，那麼您需要展開查詢的這些部分以考慮NULL值。如果其他列允許使用NULL值，則可能還需要添加其他檢查。

SELECT 
    T1.id1, 
    T1.id2, 
    T1.item1, 
    T1.item2 
FROM 
    My_Table T1 
WHERE 
    COALESCE(T1.item1, -1) = COALESCE(T1.item2, -1) OR 
    (
     T1.item1 IS NULL AND 
     NOT EXISTS 
     (
      SELECT * 
      FROM 
       My_Table T2 
      WHERE 
       T2.id2 = T1.id2 AND 
       COALESCE(T2.item1, -1) <> COALESCE(T1.item1, -1) AND 
       T2.item2 = T1.item2 
     ) 
    ) 
ORDER BY 
    T1.id2, 
    T1.id1