How to query multiple COUNT(*) with good performance - sql

Question

I have this table:
CREATE TABLE schedule (
schedule_id serial NOT NULL,
start_date date,
CONSTRAINT schedule_id PRIMARY KEY (schedule_element_id)
)
And this table:
CREATE TABLE schedule_user (
schedule_user_id serial NOT NULL,
schedule_id integer,
state int,
CONSTRAINT fk_schedule_id FOREIGN KEY (schedule_id)
REFERENCES schedule (schedule_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
)
schedule
-------------------------
|schedule_id |date |
|------------+------------|
|1 |'2013-10-10'|
|2 |'2013-10-20'|
|3 |'2013-08-13'|
-------------------------
schedule_user
-----------------------------------
|schedule_user_id|schedule_id |state|
|----------------+------------+-----|
|1 | 1 |0 |
|2 | 1 |1 |
|3 | 1 |2 |
|4 | 1 |0 |
|5 | 1 |1 |
|6 | 1 |1 |
|4 | 2 |0 |
|5 | 2 |1 |
|7 | 2 |0 |
|2 | 3 |1 |
-----------------------------------
And I want a table like this:
characteristic
---------------------------------------
|schedule_id |state0|state1|state2|total|
|------------+------+------+------+-----|
|1 |2 |3 |1 |6 |
|2 |2 |1 |0 |3 |
|3 |1 |1 |0 |2 |
---------------------------------------
I've made this query that looks as as horrible as it's performance.
SELECT
schedule.schedule_id AS id,
(( SELECT count(*) AS count
FROM schedule_user
WHERE schedule_user.schedule_id = schedule.schedule_id
AND state=0))::integer AS state0,
(( SELECT count(*) AS count
FROM schedule_user
WHERE schedule_user.schedule_id = schedule.schedule_id
AND state=1))::integer AS state1,
(( SELECT count(*) AS count
FROM schedule_user
WHERE schedule_user.schedule_id = schedule.schedule_id
AND state=2))::integer AS state2,
(( SELECT count(*) AS count
FROM schedule_user
WHERE schedule_user.schedule_id = schedule.schedule_id))::integer
AS total
FROM schedule
Is there a better way to perform such a query?
Should I create an Index to 'state' column? if so, how should it look like?

Answer 1

You want to make a pivot table. An easy way to make one in SQL if you know all of the possible values of state beforehand is using sum and case statements.
select schedule_id,
sum(case state when 0 then 1 else 0 end) as state0,
sum(case state when 1 then 1 else 0 end) as state1,
sum(case state when 2 then 1 else 0 end) as state2,
count(*) as total
from schedule_user
group by schedule_id;
Another way is to use the crosstab table function.
Neither of these will let you get away with not knowing the set of values of state (and hence the columns in the result set).

Answer 2

I would try
SELECT s.schedule_id,
COUNT(CASE WHEN su.state = 0 THEN 1 END) AS state0,
COUNT(CASE WHEN su.state = 1 THEN 1 END) AS state1,
COUNT(CASE WHEN su.state = 2 THEN 1 END) AS state2,
COUNT(su.state) AS total
FROM schedule s
LEFT
OUTER
JOIN schedule_user su
ON su.schedule_id = s.schedule_id
GROUP
BY s.schedule_id
;

Answer 3

Ths standard approach is to use SUM() with a CASE over a JOIN with a GROUP BY:
SELECT
schedule.schedule_id AS id,
SUM (case when state=0 then 1 else 0 end) AS state0,
SUM (case when state=1 then 1 else 0 end) AS state1,
SUM (case when state=2 then 1 else 0 end) AS state2,
count(*) AS total
FROM schedule
LEFT JOIN schedule_user
ON schedule_user.schedule_id = schedule.schedule_id
GROUP BY 1