Question

我想转换此表/查询

+--------+-----------------+-----------------+
| tag_id | attribute_name  | attribute_value |
+--------+-----------------+-----------------+
|      1 | tag_name        | P1001           |
|      1 | tag_address     | N7:0            |
|      1 | tag_description | Pump 1 Status   |
|      2 | tag_name        | P1002           |
|      2 | tag_address     | N7:1            |
|      2 | tag_description | Pump 2 Status   |
|      3 | tag_name        | P1003           |
|      3 | tag_address     | N7:2            |
|      3 | tag_description | Pump 3 Status   |
+--------+-----------------+-----------------+

到这个

+----------+-------------+-----------------+
| tag_name | tag_address | tag_description |
+----------+-------------+-----------------+
| P1001    | N7:0        | Pump 1 Status   |
| P1002    | N7:1        | Pump 2 Status   |
| P1003    | N7:2        | Pump 3 Status   |
+----------+-------------+-----------------+

我知道交叉表查询存在，但它迫使我为Value选择一个聚合函数。如果我选择First或Max它可行，但它不是最有效的方式。有更好的方法吗？

Answer 1

select tag_name, tag_address, tag_description from
(
    (
        select tag_id, attribute_value as tag_name
        from inputtable
        where attribute_name='tag_name'
    ) as one
    left join
    (
        select tag_id, attribute_value as tag_address
        from inputtable
        where attribute_name='tag_address'
    ) as two
    on one.tag_id=two.tag_id
)
left join
(
    select tag_id, attribute_value as tag_description
    from inputtable
    where attribute_name='tag_description'
) as three
on one.tag_id=three.tag_id

Answer 2

正如您在问题中所建议的那样，

TRANSFORM MAX(attribute_value) AS thing
SELECT tag_id
FROM your_table
GROUP BY tag_id
PIVOT attribute_name IN ('tag_name','tag_address','tag_description')

会奏效。另一种选择是

SELECT tn.tag_name, ta.tag_address, td.tag_description
FROM
    (
        (
            SELECT 
                tag_id, 
                attribute_value AS tag_name
            FROM your_table
            WHERE attribute_name = 'tag_name'
        ) AS tn
        INNER JOIN
        (
            SELECT 
                tag_id, 
                attribute_value AS tag_address
            FROM your_table
            WHERE attribute_name = 'tag_address'
        ) AS ta
            ON ta.tag_id = tn.tag_id
    )
    INNER JOIN
    (
        SELECT 
            tag_id, 
            attribute_value AS tag_description
        FROM your_table
        WHERE attribute_name = 'tag_description'
    ) AS td
        ON td.tag_id = tn.tag_id

但我可能只是选择第一个。

在MS Access中创建以下查询的最有效方法是什么？

2 个答案: