SQL query that does two GROUP BYs? - sql

Question

I'm having trouble getting the SQL for a report I need to generate. I've got the (equivalent of the) following setup:
A table articles (fields such as as name, manufacturer_id, etc).
A table sales.
FK to articles called article_id
An integer called amount
A date field
A field called type. We can assume it is a string and it can have 3 known values - 'morning', 'evening' and 'night'
I want to generate an aggregated sales report, given a start date and end date:
+--------------+---------------+--------------+-------------+
| article_name | morning_sales | evening_sales| night_sales |
+--------------+---------------+--------------+-------------+
| article 1 | 0 | 12 | 2 |
| article 2 | 80 | 3 | 0 |
... ... ... ... ...
| article n | 37 | 12 | 1 |
+--------------+---------------+--------------+-------------+
I'd like to do it as efficiently as possible. So far I've been able to generate a query that will give me one type of sale (morning, evening or night) but I'm not able to do it for multiple types simultaneously. Is it even possible?
This is what I have so far; it'll give me the article name and morning sales of all the articles in a given period - in other words, the first two columns of the report:
SELECT articles.name as article_name,
SUM(sales.amount) as morning_sales,
FROM "sales"
INNER JOIN articles ON articles.id = sales.articles_id
WHERE ( sales.date >= '2011-05-09'
AND sales.end_date <= '2011-05-16'
AND sales.type = 'morning'
)
GROUP BY sales.article_id
I guess I could do the same for evening and night, but the articles will be different; some articles might have sales in the morning but not in the evening, for example.
If I have to do 1 request per sale type, how do I "mix and match" the different article lists I'll get?
Is there a better way do do this (maybe with subqueries of some sort)?
Similarly, I'm able to build a query that gives me all the morning, evening and night sales, grouping by type. I guess my problem is that I need to do two GROUP BYs in order to get this report. I don't know how to do that, if it's possible at all.
I'm using PostgreSQL as my DB, but I'd like to stay as standard as possible. If it helps, the app using this is a Rails app.

Answer 1

Fortunately, you don't need to do multiple queries with your database format. This should work for you:
SELECT
articles.name AS article_name
SUM(IF(sales.type = 'morning', sales.amount, 0.0)) AS morning_sales,
SUM(IF(sales.type = 'evening', sales.amount, 0.0)) AS evening_sales,
SUM(IF(sales.type = 'night', sales.amount, 0.0)) AS night_sales
FROM sales
JOIN articles ON sales.article_id = articles.id
WHERE
sales.date >= "2011-01-01 00:00:00"
AND sales.date < "2011-02-01 00:00:00"
GROUP BY sales.article_id
And if there are other types, you would have to add more columns there, OR simply sum up the other types by adding this to the SELECT clause:
SUM(
IF(sales.type IS NULL OR sales.type NOT IN ('morning', 'evening', 'night'),
sales.amount, 0.0
)
) AS other_sales
The above is compatible with MySQL. To use it in Postgres, I think you'd have to change the IF expressions to CASE expressions, which should look like this (untested):
SELECT
articles.name AS article_name,
SUM(CASE WHEN sales.type = 'morning' THEN sales.amount ELSE 0.0 END) AS morning_sales,
SUM(CASE WHEN sales.type = 'evening' THEN sales.amount ELSE 0.0 END) AS evening_sales,
SUM(CASE WHEN sales.type = 'night' THEN sales.amount ELSE 0.0 END) AS night_sales
FROM sales
JOIN articles ON sales.article_id = articles.id
WHERE
sales.date >= "2011-01-01 00:00:00"
AND sales.date < "2011-02-01 00:00:00"
GROUP BY sales.article_id

Answer 2

Two options.
Option 1. A single join with computed columns for agreggation
select article_name = a.article_name ,
morning_sales = sum( case when sales.type = 'morning' then sales.amount end ) ,
evening_sales = sum( case when sales.type = 'evening' then sales.amount end ) ,
night_sales = sum( case when sales.type = 'night' then sales.amount end ) ,
other_sales = sum( case when sales.type in ( 'morning','evening','night') then null else sales.amount end ) ,
total_sales = sum( sales.amount )
from articles a
join sales s on s.articles_id = a.articles_id
where sales.date between #dtFrom and #dtThru
group by a.article_name
Option 2. multiple left joins
select article_name = a.article_name ,
morning_sales = sum( morning.amount ) ,
evening_sales = sum( evening.amount ) ,
night_sales = sum( night.amount ) ,
other_sales = sum( other.amount ) ,
total_sales = sum( total.amount )
from articles a
left join sales morning on morning.articles_id = a.articles_id
and morning.type = 'morning'
and morning.date between #dtFrom and #dtThru
left join sales evening on evening.articles_id = a.articles_id
and evening.type = 'evening'
and evening.date between #dtFrom and #dtThru
left join sales night on night.articles_id = a.articles_id
and night.type = 'evening'
and night.date between #dtFrom and #dtThru
left join sales other on other.articles_id = a.articles_id
and ( other.type is null
OR other.type not in ('morning','evening','night')
)
and other.date between #dtFrom and #dtThru
left join sales total on total.articles_id = a.articles_id
and total.date between #dtFrom and #dtThru
group by a.article_name