Converting MySQL Resultset from Rows to Columns

问题:

I have output from a select like this:

04:47:37> select * from attributes left outer join trailer_attributes on attributes.id = trailer_attributes.attribute_id;
+----+--------------+----------+-----------+------------+--------------+-----------------+
| id | name         | datatype | list_page | trailer_id | attribute_id | attribute_value |
+----+--------------+----------+-----------+------------+--------------+-----------------+
|  1 | Make         | text     |         1 |          1 |            1 | Apple           | 
|  1 | Make         | text     |         1 |          2 |            1 | sdfg            | 
|  2 | Year         | number   |         1 |          1 |            2 | 2009            | 
|  2 | Year         | number   |         1 |          2 |            2 | sdfg            | 
|  3 | Type         | text     |         0 |          1 |            3 | iPhone          | 
|  3 | Type         | text     |         0 |          2 |            3 | sdfg            | 
|  4 | Axles        | text     |         0 |          1 |            4 | asdf            | 
|  4 | Axles        | text     |         0 |          2 |            4 | sdfg            | 
|  7 | Size         | text     |         0 |          1 |            7 | asd1            | 
|  7 | Size         | text     |         0 |          2 |            7 | sdfg            | 
|  8 | Frame        | text     |         0 |          1 |            8 |                 | 
|  8 | Frame        | text     |         0 |          2 |            8 | sdfg            | 
|  9 | Height       | text     |         0 |          1 |            9 |                 | 
|  9 | Height       | text     |         0 |          2 |            9 | sdfg            | 
| 10 | Dollies      | text     |         0 |          1 |           10 |                 | 
| 10 | Dollies      | text     |         0 |          2 |           10 | sdfg            | 
| 11 | Tires/Wheels | text     |         0 |          1 |           11 |                 | 
| 11 | Tires/Wheels | text     |         0 |          2 |           11 | sdfg            | 
| 12 | Condition    | text     |         1 |          1 |           12 | New             | 
| 12 | Condition    | text     |         1 |          2 |           12 | sdfg            | 
| 13 | Title        | text     |         0 |          1 |           13 |                 | 
| 13 | Title        | text     |         0 |          2 |           13 | sdfg            | 
+----+--------------+----------+-----------+------------+--------------+-----------------+

I want to convert it to something more along the lines of:

id, Make,  Year, Type,   Axles, Size, Frame (etc)
1,  Apple, 2009, iPhone, .....
2,  sdfg,  sdfg, sdfg,   .....

Any suggestions?

回答1:

Mmmm...EAVs. One of the many reasons to avoid EAVs (entity-attribute_value) is that they are harder to report and query against. However, if the attributes you want are known ahead of time, you can do something like:

Select id
    , Min( Case When name = 'Make' Then attribute_value End ) As Make
    , Min( Case When name = 'Year' Then attribute_value End ) As Year
    , Min( Case When name = 'Type' Then attribute_value End ) As Type
    , Min( Case When name = 'Axles' Then attribute_value End ) As Axles
    , Min( Case When name = 'Size' Then attribute_value End ) As Size
    , Min( Case When name = 'Frame' Then attribute_value End ) As Frame
    , ...
From attributes
Where name In('Make','Year','Type','Axles','Size','Frame',....)
Group By id

Now, MySQL, does have a GROUP_CONCAT which will let you concatenate multiple values for the same attribute into a list if you allow that (e.g. if an entity can have multiple Make attributes).

回答2:

This may not be an option for you, but ideally you should convert each attribute into a column of the main table. Relational databases are designed to handle attributes as columns, not rows. Therefore they perform much better when you use them like that, and the SQL becomes much simpler too.