SQL question with attempt on customer information - sql

Question

Schema
Question: List all paying customers with users who had 4 or 5 activities during the week of February 15, 2021; also include how many of the activities sent were paid, organic and/or app store. (i.e. include a column for each of the three source types).
My attempt so far:
SELECT source_type, COUNT(*)
FROM activities
WHERE activity_time BETWEEN '02-15-21' AND '02-19-21'
GROUP BY source_type
I would like to get a second opinion on it. I didn't include the accounts table because I don't believe that I need it for this query, but I could be wrong.

Answer 1

Have you tried to run this? It doesn't satisfy the brief on FOUR counts:
List all the ... customers (that match criteria)
There is no customer information included in the results at all, so this is an outright fail.
paying customers
This is the top level criteria, only customers that are not free should be included in the results.
Criteria: users who had 4 or 5 activities
There has been no attempt to evaluate this user criteria in the query, and the results do not provide enough information to deduce it.
there is further ambiguity in this requirement, does it mean that it should only include results if the account has individual users that have 4 or 5 acitvities, or is it simply that the account should have 4 or 5 activities overall.
If this is a test question (clearly this is contrived, if it is not please ask for help on how to design a better schema) then the use of the term User is usually very specific and would suggest that you need to group by or otherwise make specific use of this facet in your query.
Bonus: (i.e. include a column for each of the three source types).
This is the only element that was attempted, as the data is grouped by source_type but the information cannot be correlated back to any specific user or customer.
Next time please include example data and the expected outcome with your post. In preparing the data for this post you would have come across these issues yourself and may have been inspired to ask a different question, or through the process of writing the post up you may have resolved the issue yourself.
without further clarification, we can still start to evolve this query, a good place to start is to exclude the criteria and focus on the format of the output. the requirement mentions the following output requirements:
List Customers
Include a column for each of the source types.
Firstly, even though you don't think you need to, the request clearly states that Customer is an important facet in the output, and in your schema account holds the customer information, so although we do not need to, it makes the data readable by humans if we do include information from the account table.
This is a standard PIVOT style response then, we want a row for each customer, presenting a count that aggregates each of the values for source_type. Most RDBMS will support some variant of a PIVOT operator or function, however we can achieve the same thing with simple CASE expressions to conditionally put a value into projected columns in the result set that match the values we want to aggregate, then we can use GROUP BY to evaluate the aggregation, in this case a COUNT
The following syntax is for MS SQL, however you can achieve something similar easily enough in other RBDMS
OP please tag this question with your preferred database engine...
NOTE: there is NO filtering in this query... yet
SELECT accounts.company_id
, accounts.company_name
, paid = COUNT(st_paid)
, organic = COUNT(st_organic)
, app_store = COUNT(st_app_store)
FROM activities
INNER JOIN accounts ON activities.company_id = accounts.company_id
-- PIVOT the source_type
CROSS APPLY (SELECT st_paid = CASE source_type WHEN 'paid' THEN 1 END
,st_organic = CASE source_type WHEN 'organic' THEN 1 END
,st_app_store = CASE source_type WHEN 'app store' THEN 1 END
) as PVT
GROUP BY accounts.company_id, accounts.company_name
This results in the following shape of result:
company_id
company_name
paid
organic
app_store
apl01
apples
4
8
0
ora01
oranges
6
12
0
Criteria
When you are happy with the shpe of the results and that all the relevant information is available, it is time to apply the criteria to filter this data.
From the requirement, the following criteria can be identified:
paying customers
The spec doesn't mention paying specifically, but it does include a note that (free customers have current_mrr = 0)
Now aren't we glad we did join on the account table :)
users who had 4 or 5 activities
This is very specific about explicitly 4 or 5 activities, no more, no less.
For the sake of simplicity, lets assume that the user facet of this requirement is not important and that is is simply a reference to all users on an account, not just users who have individually logged 4 or 5 activities on their own - this would require more demo data than I care to manufacture right now to prove.
during the week of February 15, 2021.
This one was correctly identified in the original post, but we need to call it out just the same.
OP has used Monday to Friday of that week, there is no mention that weeks start on a Monday or that they end on Friday but we'll go along, it's only the syntax we need to explore today.
In the real world the actual values specified in the criteria should be parameterised, mainly because you don't want to manually re-construct the entire query every time, but also to sanitise input and prevent SQL injection attacks.
Even though it seems overkill for this post, using parameters even in simple queries helps to identify the variable elements, so I will use parameters for the 2nd criteria to demonstrate the concept.
DECLARE #from DateTime = '2021-02-15' -- Date in ISO format
DECLARE #to DateTime = (SELECT DateAdd(d, 5, #from)) -- will match Friday: 2021-02-19
/* NOTE: requirement only mentioned the start date, not the end
so your code should also only rely on the single fixed start date */
SELECT accounts.company_id, accounts.company_name
, paid = COUNT(st_paid), organic = COUNT(st_organic), app_store = COUNT(st_app_store)
FROM activities
INNER JOIN accounts ON activities.company_id = accounts.company_id
-- PIVOT the source_type
CROSS APPLY (SELECT st_paid = CASE source_type WHEN 'paid' THEN 1 END
,st_organic = CASE source_type WHEN 'organic' THEN 1 END
,st_app_store = CASE source_type WHEN 'app store' THEN 1 END
) as PVT
WHERE -- paid accounts = exclude 'free' accounts
accounts.current_mrr > 0
-- Date range filter
AND activity_time BETWEEN #from AND #to
GROUP BY accounts.company_id, accounts.company_name
-- The fun bit, we use HAVING to apply a filter AFTER the grouping is evaluated
-- Wording was explicitly 4 OR 5, not BETWEEN so we use IN for that
HAVING COUNT(source_type) IN (4,5)

Answer 2

I believe you are missing some information there.
without more information on the tables, I can only guess that you also have a customer table. i am going to assume there is a customer_id key that serves as key between both tables
i would take your query and do something like:
SELECT customer_id,
COUNT() AS Total,
MAX(CASE WHEN source_type = "app" THEN "numoperations" END) "app_totals"),
MAX(CASE WHEN source_type = "paid" THEN "numoperations" END) "paid_totals"),
MAX(CASE WHEN source_type = "organic" THEN "numoperations" END) "organic_totals"),
FROM (
SELECT source_type, COUNT() AS num_operations
FROM activities
WHERE activity_time BETWEEN '02-15-21' AND '02-19-21'
GROUP BY source_type
) tb1 GROUP BY customer_id
This is the most generic case i can think of, but does not scale very well. If you get new source types, you need to modify the query, and the structure of the output table also changes. Depending on the sql engine you are using (i.e. mysql vs microsoft sql) you could also use a pivot function.
The previous query is a little bit rough, but it will give you a general idea. You can add "ELSE" statements to the clause, to zero the fields when they have no values, and join with the customer table if you want only active customers, etc.