Question

I have a Person table and an attribute table. An Person can have 0 or more attributes, so I have this relationship stored in an intermediary key-value table, eg

------------         --------------       ---------------
|Person     |        |key val tbl |       | attribute   |
 -----------         --------------       ---------------
|p_id|p_name|        | p_id |a_id |       | a_id |a_name|
| 1  |simon |        |  1   | 1   |       |  1   | tall |
| 2  |eric  |        |  1   | 2   |       |  2   | cool |
                     |  2   | 2   |

How could I write a query to return something like this?

|name |attr1|attr2|  
|simon|tall |cool |
|eric |     |cool |

There are only ~6 attributes and that will not likely change.

Thanks!

Answer 1

You need to join and aggregate:

select p.name,
       max(case when a.a_id = 1 then a.a_name end) as attr1,
       max(case when a.a_id = 2 then a.a_name end) as attr2
from person p join
     keyval kv
     on p.key = kv.key join
     attribute a
     on kv.a_id = a.a_id
group by p.name

This query joins the three tables together, which will produce a table with one row for each person/attribute pair. The final group by aggregates at the person level, “pivoting” the attributes across the row. The max(case …) expression simply chooses the value of attribute for column .