Select multiple row values into single row with multi-table clauses

Question

I've searched the forums and while I see similar posts, they only address pieces of the full query I need to formulate (array_aggr, where exists, joins, etc.). If the question I'm posting has been answered, I will gladly accept references to those threads.

I did find this thread ...which is very similar to what I need, except it is for MySQL, and I kept running into errors trying to get it into psql syntax. Hoping someone can help me get everything together. Here's the scenario:

Attribute

attrib_id | attrib_name

UserAttribute

user_id | attrib_id | value

Here's a small example of what the data looks like:

Attribute

attrib_id | attrib_name
-----------------------
1         | attrib1
2         | attrib2
3         | attrib3
4         | attrib4
5         | attrib5

UserAttribute -- there can be up to 15 attrib_id's/value's per user_id

user_id | attrib_id | value
----------------------------
101     | 1         | valueA
101     | 2         | valueB
102     | 1         | valueC
102     | 2         | valueD
103     | 1         | valueA
103     | 2         | valueB
104     | 1         | valueC
104     | 2         | valueD
105     | 1         | valueA
105     | 2         | valueB

Here's what I'm looking for

Result

user_id    | attrib1_value | attrib2_value
--------------------------------------------------------
101        | valueA        | valueB
102        | valueC        | valueD
103        | valueA        | valueB
104        | valueC        | valueD
105        | valueA        | valueB

As shown, I'm looking for single rows that contain: - user_id from the UserAttribute table - attribute values from the UserAttribute table

Note: I only need attribute values from the UserAttribute table for two specific attribute names in the Attribute table

Again, any help or reference to an existing solution would be greatly appreciated.

---

UPDATE:

@ronin provided a query that gets the results desired:

SELECT ua.user_id
      ,MAX(CASE WHEN a.attrib_name = 'attrib1' THEN ua.value ELSE NULL END) AS attrib_1_val
      ,MAX(CASE WHEN a.attrib_name = 'attrib2' THEN ua.value ELSE NULL END) AS attrib_2_val
  FROM UserAttribute ua
  JOIN Attribute a ON (a.attrib_id = ua.attrib_id)
  WHERE a.attrib_name IN ('attrib1', 'attrib2')
  GROUP BY ua.user_id;

To build on that, I tried to add some 'LIKE' pattern matching within the 'WHEN' condition (against the ua.value), but everything ends up as the 'FALSE' value. Will start a new question to see if that can be incorporated if I cannot figure it out. Thanks all for the help!!

Answer 1

If each attribute only has a single value for a user, you can start by making a sparse matrix:

SELECT user_id
      ,CASE WHEN attrib_id = 1 THEN value ELSE NULL END AS attrib_1_val
      ,CASE WHEN attrib_id = 2 THEN value ELSE NULL END AS attrib_2_val
  FROM UserAttribute;

Then compress the matrix using an aggregate function:

SELECT user_id
      ,MAX(CASE WHEN attrib_id = 1 THEN value ELSE NULL END) AS attrib_1_val
      ,MAX(CASE WHEN attrib_id = 2 THEN value ELSE NULL END) AS attrib_2_val
  FROM UserAttribute
  GROUP BY user_id;

In response to the comment, searching by attribute name rather than id:

SELECT ua.user_id
      ,MAX(CASE WHEN a.attrib_name = 'attrib1' THEN ua.value ELSE NULL END) AS attrib_1_val
      ,MAX(CASE WHEN a.attrib_name = 'attrib2' THEN ua.value ELSE NULL END) AS attrib_2_val
  FROM UserAttribute ua
  JOIN Attribute a ON (a.attrib_id = ua.attrib_id)
  WHERE a.attrib_name IN ('attrib1', 'attrib2')
  GROUP BY ua.user_id;

Answer 2

Starting with Postgres 9.4 you can use the simpler aggregate FILTER clause:

SELECT user_id
      ,MAX(value) FILTER (WHERE attrib_id = 1) AS attrib_1_val
      ,MAX(value) FILTER (WHERE attrib_id = 2) AS attrib_2_val
FROM   UserAttribute
WHERE  attrib_id IN (1,2)
GROUP  BY 1;

For more than a few attributes or for top performance, look to crosstab() from the additional module tablefunc (Postgres 8.3+). Details here:

• PostgreSQL Crosstab Query

Answer 3

What about something like this:

select ua.user_id, a.attrib_name attrib_value1, a2.attrib_name attrib_value2
from user_attribute ua
left join attribute a on a.atribute_id=ua.attribute_id and a.attribute_id in (1,2)
left join user_attribute ua2 on ua2.user_id=ua.user_id and ua2.attribute_id > ua.attribute_id
left join attribute a2 on a2.attribute_id=ua2.attribute_id and a2.attribute_id in (1,2)