Postgres COUNT number of column values with INNER JOIN - database

Question

I am creating a report in Postgres 9.3. This is my SQL Fiddle.
Basically I have two tables, responses and questions, the structure is:
responses
->id
->question_id
->response
questions
->id
->question
->costperlead
for the column response there can only be 3 values, Yes/No/Possbily,
and my report should have the columns:
question_id
, # of Yes Responses
, # of No Responses
, # of Possbily Responses
, Revenue
Then:
# of Yes Responses - count of all Yes values in the response column
# of No Responses - count of all No values in the response column
# of Possbily Responses - count of all 'Possbily' values in the response column
Revenue is the costperlead * (Number of Yes Responses + Number of Possibly Responses).
I don't know how to construct the query, I'm new plus I came from MySQL so some things are different for postgres. In my SQL Fiddle sample most responses are Yes and Null, it's ok eventually, there will be Possibly and No.
So far I have only:
SELECT a.question_id
FROM responses a
INNER JOIN questions b ON a.question_id = b.id
WHERE a.created_at = '2015-07-17'
GROUP BY a.question_id;

Answer 1

You should try:
SELECT a.question_id,
SUM(CASE WHEN a.response = 'Yes' THEN 1 ELSE 0 END) AS NumsOfYes,
SUM(CASE WHEN a.response = 'No' THEN 1 ELSE 0 END) AS NumsOfNo,
SUM(CASE WHEN a.response = 'Possibly' THEN 1 ELSE 0 END) AS NumOfPossibly,
costperlead * SUM(CASE WHEN a.response = 'Yes' THEN 1 ELSE 0 END) + SUM(CASE WHEN a.response = 'Possibly' THEN 1 ELSE 0 END) AS revenue
FROM responses a
INNER JOIN questions b ON a.question_id = b.id
GROUP BY a.question_id, b.costperlead

Answer 2

Since the only predicate filters rows from table responses, it would be most efficient to aggregate responses first, then join to questions:
SELECT *, q.costperlead * (r.ct_yes + r.ct_maybe) AS revenue
FROM (
SELECT question_id
, count(*) FILTER (WHERE response = 'Yes') AS ct_yes
, count(*) FILTER (WHERE response = 'No') AS ct_no
, count(*) FILTER (WHERE response = 'Possibly') AS ct_maybe
FROM responses
WHERE created_at = '2015-07-17'
GROUP BY 1
) r
JOIN questions q ON q.id = r.question_id;
db<>fiddle here
This uses the aggregate FILTER clause (in Postgres 9.4 or later). See:
Aggregate columns with additional (distinct) filters
Aside: consider implementing response as boolean type with true/false/null.
For Postgres 9.3:
SELECT *, q.costperlead * (r.ct_yes + r.ct_maybe) AS revenue
FROM (
SELECT question_id
, count(response = 'Yes' OR NULL) AS ct_yes
, count(response = 'No' OR NULL) AS ct_no
, count(response = 'Possibly' OR NULL) AS ct_maybe
FROM responses
WHERE created_at = '2015-07-17'
GROUP BY 1
) r
JOIN questions q ON q.id = r.question_id;
Old sqlfiddle
Comprehensive comparison of techniques:
For absolute performance, is SUM faster or COUNT?