SQL query to count number of checkins per month - sql

Question

To put a long story short, I am working on a database using PostgreSQL that is managing yelp checkins. The checkintable has the attributes business_id(string), date(string in form yyyy-mm-dd), and time(string in form 00:00:00).
What I simply need to do is, given a business_id, I need to return a list of the total number of checkins based on just the mm (month) value.
So for instance, I need to retrieve the total checkins that were in Jan, Feb, March, April, etc, not based upon the year.
Any help is greatly appreciated. I've already considered group by clauses but I didn't know how to factor in '%mm%'.

Answer 1

Reiterating Gordon, class or not, storing dates and times as strings makes things harder, slower, and more likely to break. It's harder to take advantage of Postgres's powerful date math functions. Storing dates and times separately makes things even harder; you have to concatenate them together to get the full timestamp which means it will not be indexed. Determining the time between two events becomes unnecessarily difficult.
It should be a single timestamp column. Hopefully your class will introduce that shortly.
What I simply need to do is, given a business_id, I need to return a list of the total number of checkins based on just the mm (month) value.
This is deceptively straightforward. Cast your strings to dates, fortunately they're in ISO 8601 format so no reformatting is required. Then use extract to extract just the month part.
select
extract('month' from checkin_date::date) as month,
count(*)
from yelp_checkins
where business_id = ?
group by month
order by month
But there's a catch. What if there are no checkins for a business on a given month? We'll get no entry for that month. This is a pretty common problem.
If we want a row for every month, we need to generate a table with our desired months with generate_series, then left join with our checkin table. A left join ensures all the months (the "left" table) will be there even if there is no corresponding month in the join table (the "right" table).
select
months.month,
count(business_id)
from generate_series(1,12) as months(month)
left join yelp_checkins
on months.month = extract('month' from checkin_date::date)
and business_id = ?
group by months.month
order by months.month
Now that we have a table of months, we can group by that. We can't use a where business_id = ? clause or that will filter out empty months after the left join has happened. Instead we must put that as part of the left join.
Try it.

Answer 2

Why would you store the date as a string? That is a broken data model. You should fix the data.
That said, I recommend converting a date and truncating to the first day of the month:
select date_trunc('day', datestr::date) as yyyymm, count(*)
from t
group by yyyymm
order by yyyymm;
If you don't want these based on the year, then use extract():
select extract(month from datestr::date) as mm, count(*)
from t
group by mm
order by mm;