Re: count(boolean)

SELECT question_id,         COUNT(*)    FROM Ansers_Boolean   WHERE value=TRUEGROUP BY question_id;


sort of works; it's fast & correct, but if a question has no true responses,
it leaves off that question. This may or may not be acceptable to you.


You could fix that with:
  SELECT question_id,         COUNT(*)    FROM Answers_Boolean   WHERE value=TRUEGROUP BY question_id

UNION ALL
  SELECT question_id,         0    FROM Answers_Boolean AS AB0   WHERE NOT EXISTS (SELECT *                       FROM
Answers_BooleanAS AB1                      WHERE AB0.question_id = AB1.question_id                        AND value =
TRUE);


But it performs slowly. I normally create turn the bools into 1 or 0 and add
them up (called a "Characteristic function"):
 SELECT question_id,        SUM(CASE value WHEN TRUE THEN 1 ELSE 0 END) AS num_true   FROM Answers_Boolean
GROUP BY question_id;

Plus, it's easy this way to add a num_false column, by just copying
num_true.


Joel BURTON | joel@joelburton.com | joelburton.com | aim: wjoelburton
Knowledge Management & Technology Consultant

> -----Original Message-----
> From: pgsql-sql-owner@postgresql.org
> [mailto:pgsql-sql-owner@postgresql.org]On Behalf Of Dan MacNeil
> Sent: Sunday, May 19, 2002 10:17 AM
> To: pgsql-sql
> Subject: [SQL] count(boolean)
>
>
> I have a table answers_boolean:
>
>  question_id   | integer   | not null
>  evaluation_id | integer   | not null
>  value             | boolean |
>
> I'd like output in the form:
>
> question_id,  count_true, count_false
>
> ....where count_true is the number of questions answered "true"
>
> SELECT
>     question_id AS id, value AS val , count(value) AS cnt
> FROM
>      answers_boolean
> GROUP BY
>      question_id,value;
>
> gives me:
>  id | val | cnt
> ----+-----+-----
>   2 | f   |   3
>   2 | t   |   3
>   3 | f   |   2
>   3 | t   |   4
>
> ...which is sorta what I want.
>
> I've looked through the docs & archive but haven't found the answer.
>
> TIA
>
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