SQL query to get values by the calendar month - sql

Question

I have the following schema:
CREATE TABLE phone_clicks (
id integer NOT NULL,
start_date date NOT NULL,
industry_id integer NOT NULL,
clicks integer DEFAULT 0 NOT NULL
);
insert into phone_clicks(id, start_date, industry_id)
values
(1, '2021-10-03', 1),
(2, '2021-10-03', 2),
(3, '2021-10-02', 3),
(4, '2021-10-01', 1),
(5, '2021-11-01', 3),
(6, '2021-11-01', 4),
(7, '2021-11-02', 1),
(8, '2021-11-02', 2);
and i want to get a return object that has the industry id along with the number of items but based on the calendar month for the previous two months, not including the current. So i want the number of items in October and the number in November.
What i have, that is not working
SELECT industry_id,
sum(
CASE
WHEN start_date >= (date_trunc('month', start_date) - interval '1 month')::date AND start_date < (date_trunc('month', start_date) - interval '2 month')::date THEN 1
ELSE 0
END) AS last_month,
sum(
CASE
WHEN start_date >= (date_trunc('month', start_date) - interval '2 month')::date AND start_date <= (date_trunc('month', start_date) - interval '3 month')::date THEN 1
ELSE 0
END) AS prev_month
FROM phone_clicks
Group by industry_id
What i am currently getting from the above :
industry_id last_month prev_month
4 0 0
1 0 0
3 0 0
2 0 0
What i am expecting:
industry_id last_month prev_month
4 1 0
1 1 2
3 1 1
2 1 1
So industry 1, it has two items with start_date in October and 1 in November.

Answer 1

date_trunc('month', ...) always returns the first of the month, so you don't really need the >= .. <= in your CASE expression.
It's enough to compare the month start of "start_date" with the month start of last month, which is date_trunc('month', current_date - interval '1 month')
The query can also be simplified a bit by using the filter operator.
SELECT industry_id,
count(*) filter (where date_trunc('month', start_date) = date_trunc('month', current_date - interval '1 month')) as last_month,
count(*) filter (where date_trunc('month', start_date) = date_trunc('month', current_date - interval '2 month')) as prev_month
FROM phone_clicks
group by industry_id
order by industry_id