SQL: Turning Rows into Columns for Variable Number of Rows - sql

Question

I have two tables whose simplified structure looks like this:
RESPONSES
id
created
ACCESSORY VALUES
id
response_id
sensor_id
value
I want to create a view that flattens all accessory values for a given response into one row over a time period (filtering on response.created). I think I want a Pivot table or Crosstab, but I'm unfamiliar with both and the examples I've found mainly deal with a known number of columns. To complicate things, a given sensor could only appear for part of the time period if a user started or stopped tracking it during the time in question. Ideally I'd hold a NULL in that column for the sensor in any rows when it was not present. Is this possible? If so, am I on the right track or looking in the wrong place?
The SQL to get the data as individual rows looks like
SELECT r.id, a.sensor_id, a.value from results_response r
INNER JOIN results_accessoryvalue a ON r.id = a.response_id
WHERE r.installation_id = 40
AND r.created BETWEEN '2013-04-01' AND '2013-05-01'
ORDER BY r.created
but I'm not having any luck trying to use it in a crosstab because I don't know how to specify dynamic columns.

Answer 1

you should use crosstab with some additons. i had this problem too and solved it for my proposal like this.
first install crosstab extension
the trick i used is to create 2 additional functions. one to get the type information, needed as resultset for crosstab function. look at this:
CREATE OR REPLACE FUNCTION "TEMPORARY.TYPE.FROM.COLUMN"(text, text)
RETURNS text AS
$BODY$
DECLARE
typestring TEXT;
returnrec RECORD;
BEGIN
typestring := '';
FOR returnrec IN EXECUTE $1 LOOP
typestring := typestring||', "'||returnrec."Column"||'" '||$2;
END LOOP;
RETURN typestring;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
Now we can write our second function:
CREATE OR REPLACE FUNCTION "DYNAMIC.CROSSTAB"(text, text, text)
RETURNS text AS
$BODY$
DECLARE
typestring TEXT;
executestring TEXT;
BEGIN
DROP TABLE IF EXISTS "TBL.Crosstab";
SELECT "REPORTING"."TEMPORARY.TYPE.FROM.COLUMN"($2,$3) INTO typestring;
executestring := 'CREATE TEMPORARY TABLE "TBL.Crosstab" AS (SELECT * FROM crosstab('''||$1||''','''||$2||''') AS (row_name DATE'||typestring||'));';
EXECUTE executestring;
RETURN '"TBL.Crosstab"';
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
To create your crosstab simply call:
SELECT * FROM "DYNAMIC.CROSSTAB"(text, text, text);
It returns the name of an temporary table for your session filled with your result. I needed it this way.
Parameter explanation:
First = The query to get your data (must have 3 columns: row_name, cat and value)
Second = The query to get your columns wich returns all your categories (cat)
Third = The columntype for our temporary type
it's not perfect but fit's our needs. we have statements, fetching more than 450 coloumns and some thounds rows at this way. hope it helps

Answer 2

This is an alternative approach using PostgreSQL arrays
create table responses (
id serial not null unique,
x integer,
created timestamp default now()
);
create table accessory (
id serial not null,
responses_id integer references responses (id),
sensor_id integer,
value text,
created timestamp default now()
);
insert into responses (x) values ( 1), (2),(3),(4);
insert into accessory (responses_id , sensor_id, value ) values
( 1, 1, 'A' ), ( 1, 2, 'Ab' ), ( 1, 3, 'Ac' ), ( 1, 4, 'Ad' ),
( 2, 4, 'Ab' ), ( 1, 2, 'bAb' ), ( 3, 3, 'bAc' ), ( 4, 4, 'bAd' );
select *, array(
select value
from accessory
where accessory.responses_id = responses.id
order by accessory.created
) as accessory_values
from responses;
Query result includes an array column with all the accessory values that match response.id