count boolean column, and average another column based on boolean column - sql

Question

CREATE TABLE test (
calculate_time int4 NULL,
status bool NULL
);
INSERT INTO test (calculate_time,status) VALUES
(10,true)
,(15,true)
,(20,true)
,(20,true)
,(5,false)
,(10,false)
,(15,false)
,(100,NULL)
,(200,NULL)
,(300,NULL)
;
With this query it average all calculated_time values. Is there a way I can tell it only average ones where status = true? I tried adding a where clause but would make failed and suspended result in 0.
select
avg(calculate_time) as cal_time,
count(case when status = true then 1 end) as completed,
count(case when status = false then 1 end) as failed,
count(case when status is null then 1 end) as suspended
from test;

Answer 1

You seem to understand the concept of conditional aggregation. You can just also use a CASE expression for the average as you did for the other terms in your select:
select
avg(case when status then calculate_time end) as cal_time,
count(case when status then 1 end) as completed,
count(case when not status then 1 end) as failed,
count(case when status is null then 1 end) as suspended
from test;
This works because the AVG function, like most of the other aggregate functions, ignore NULL values. So the records for which status is not true, their calculate_time values would be effectively ignored and would not influence the overall average.
Other side note: You may use boolean values in a Postgres query directly without comparing them to true. That is, the following two CASE expressions are equivalent, with the second one being less terse:
avg(case when status = true then calculate_time end) as cal_time,
avg(case when status then calculate_time end) as cal_time,

Answer 2

Adding to #Tim's answer, since Postgres 9.4 you can add a filter clause to aggregate function calls, which may save you some of the boiler-plate of writing your own case expressions:
select
avg(calculate_time) filter (where status) as cal_time,
count(*) filter (where status) as completed,
count(*) filter (where not status) as failed,
count(*) filter (where status is null) as suspended
from test;