Repeat Customers with multiple purchases on the same day counts a 1 - sql-server

Question

I am trying to wrap my head around this problem. I was asked to create a report that show repeat customers in our database.
One of the requirements is if a customer has more than 1 order on a specific date, it would only count as 1.
Then if they have more than 1 purchase date, they would then count as a repeat customer.
Searching on here, I found this which works for finding the Customers with more then 1 purchase on a specific purchase date.
SELECT DISTINCT s.[CustomerName], s.PurchaseDate
FROM Reports.vw_Repeat s WHERE s.PurchaseDate <> ''
GROUP BY s.[CustomerName] , cast(s.PurchaseDate as date)
HAVING COUNT(*) > 1;
This MSSQL code works like it should, by showing customers who had more than 1 purchase on the same date.
My problem is what would the best approach be to Join this into another query (this is where i need help) that then shows a complete repeat customer list where customers with more than 1 purchase would be returned.
I am using MSSQL. Any help would be greatly appreciated.

Answer 1

You're close, you need to move distinct into your having clause because you want to include only customers that have more than 1 distinct purchase date.
Also, only group by the customer id because the different dates have to be part of the same group for count distinct to work.
SELECT s.[CustomerName], COUNT(distinct cast(s.PurchaseDate as date))
FROM Reports.vw_Repeat s WHERE s.PurchaseDate <> ''
GROUP BY s.[CustomerName]
HAVING COUNT(distinct cast(s.PurchaseDate as date)) > 1;

Answer 2

If you want to pass a parameter to a query and join the result, that's what table-valued functions are for. When you join it, you use CROSS APPLY or OUTER APPLY instead of an INNER JOIN or a LEFT JOIN.
Also, I think this goes without saying, but when you check if PurchaseDate is empty:
WHERE s.PurchaseDate <> ''
Could be issues there... it implies it's a varchar field instead of a datetime (yes?) and doesn't handle null values. You might, at least, want to replace that with ISNULL(s.PurchaseDate, '') <> ''. If it's actually a datetime, use IS NOT NULL instead of <> ''.
(Edited to add sample data and DDL statements. I recommend adding these to SQL posts to assist answerers. Also, I made purchasedate a varchar instead of a datetime because of the string comparison in the query.)
https://technet.microsoft.com/en-us/library/ms191165(v=sql.105).aspx
CREATE TABLE company (company_name VARCHAR(25))
INSERT INTO company VALUES ('Company1'), ('Company2')
CREATE TABLE vw_repeat (customername VARCHAR(25), purchasedate VARCHAR(25), company VARCHAR(25))
INSERT INTO vw_repeat VALUES ('Cust1', '11/16/2017', 'Company1')
INSERT INTO vw_repeat VALUES ('Cust1', '11/16/2017', 'Company1')
INSERT INTO vw_repeat VALUES ('Cust2', '11/16/2017', 'Company2')
CREATE FUNCTION [dbo].tf_customers
(
#company varchar(25)
)
RETURNS TABLE AS RETURN
(
SELECT s.[CustomerName], cast(s.PurchaseDate as date) PurchaseDate
FROM vw_Repeat s
WHERE s.PurchaseDate <> '' AND s.Company = #company
GROUP BY s.[CustomerName] , cast(s.PurchaseDate as date)
HAVING COUNT(*) > 1
)
GO
SELECT *
FROM company c
CROSS APPLY tf_customers(c.company_name)

Answer 3

First thanks to everyone for the help.
#MaxSzczurek suggested I use table-valued functions. After looking into this more, I ended up using just a temporary table first to get the DISTINCT purchase dates for each Customer. I then loaded that into another temp table RIGHT JOINED to the main table. This gave me the result I was looking for. Its a little(lot) ugly, but it works.