Question

I don’t quite know how I should describe the problem for title, but here’s my question.
I have a table named hello with two columns named time and state.

Time | State

Here’s an example of the data I have

1 DC
1 VA
1 VA
2 DC
2 MD
3 MD
3 MD
3 VA
3 DC

I would like to get all the possible time and the count of “VA” (0 if “VA” doesn’t appear at the time)
The output would look like this

Time Number
1 2
2 0
3 1

I tried to do

SELECT DISTINCT time,
    COUNT(state) as Number
        FROM hello
        WHERE state = 'VA'
        GROUP BY time

but it doesn’t seem to work.

Answer 1

This is a conditional aggregation:

select time, sum(case when state = 'VA' then 1 else 0 end) as NumVA
from hello
group by time

I want to add that you should never use distinct when you have a group by. The two are redundant. Distinct as a keyword is not even needed in the SQL language; semantically, it is just shorthand for grouping by all the columns.