Question

我的查询看起来像：

SELECT *,
       (SELECT Attribute FROM TableOfAttributes WHERE KeyField = MyTable.KeyField AND Type = "A") AS Attribute1,
       (SELECT Attribute FROM TableOfAttributes WHERE KeyField = MyTable.KeyField AND Type = "B") AS Attribute2,
       (SELECT Attribute FROM TableOfAttributes WHERE KeyField = MyTable.KeyField AND Type = "C") AS Attribute3
FROM
       MyTable

确实！在MyTable中，信息是水平的，但在TableOfAttributes中它是垂直的，我正在试图弄清楚如何破坏这些嵌套查询，因为目前执行时间太长（超过一个小时）。

总结一下：我有一个带有条目的表，每个条目在另一个表中都有属性，每个属性都存储在一个记录中，一个条目有3个属性。

我想出现：

[Entry ID] [Entry Something] [Attribute1] [Attribute2] [Attribute3]

你们怎么解决这个问题？

提前致谢

Miloud B。

Answer 1

您可以使用每个表的别名加入表。如果有一个Att表中没有匹配的实例，则需要外连接。

SELECT m.*, Att1.Attribute, Att2.Attribute, Att3.Attribute 
FROM MyTable m, TableOfAttributes Att1, TableOfAttributes Att2, TableOfAttributes Att3 
WHERE Att1.KeyField = m.KeyField AND Type = "A" 
and Att2.KeyField = m.KeyField AND Type = "B"
and Att3.KeyField = m.KeyField AND Type = "C"

Answer 2

您还可以使用JOIN表格只执行一次TableOfAttribute，然后执行GROUP BY。

SELECT  [Entry ID], [Entry Something], 
        MIN(CASE WHEN [Type] = 'A' THEN Attribute ELSE NULL END) AS Attribute1,
        MIN(CASE WHEN [Type] = 'B' THEN Attribute ELSE NULL END) AS Attribute2,
        MIN(CASE WHEN [Type] = 'C' THEN Attribute ELSE NULL END) AS Attribute3
FROM MyTable 
LEFT JOIN TableOfAttributes
ON MyTable.KeyField = TableOfAttributes.KeyField
GROUP BY [Entry ID], [Entry Something]

Access 2003 Nested选择itche

2 个答案: