Question

I am running PostgreSQL. I have a table called foo.Its content are:

city|number
'oo'|12
'ss'|11
'ss'|23
'oo'|11
'pp'|21

If I run a query like select count(city) from foo group by city having number<21 I will get

city|number
'oo'|2
'ss'|2

but I want the result to consider all the possible cases of city like this:

city|number
'oo'|2
'ss'|2
'pp'|0

How should the query look like?

Answer 1

This is could be solved with a left join:

select f1.city, count(f2.number) as total from foo as f1
left join foo as f2
on f1.cityId = f2.cityId and f1.number < 21
group by f1.city

Here is a working example

PS: Your example is wrong because ss should have a value of 1, not 2:

YOUR EXAMPLE         WHAT I THINK YOU REALLY WANT
city|number                   city|number
'oo'|2                        'oo'|2
'ss'|2                        'ss'|1
'pp'|0                        'pp'|0 

Hope this helps.