Access 2003 嵌套选择痒

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 中是垂直的，我正在尝试弄清楚如何撕掉这些嵌套查询，因为目前执行时间太长（超过一个小时）。

总结一下：我有一张表，里面有条目，每个条目都有另一个表中的属性，每个属性都存储在一条记录中，一个条目有3个属性。

我想出现：

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

你们会怎么解决呢？

提前致谢

米卢德 B.

Answer 1

您可能只是使用每个表的别名来连接表。如果在某些情况下，其中一个 Att 表中不存在匹配项，则您将需要外连接。

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

您也可以只对TableOfAttribute 表执行一个JOIN，然后执行一个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]