Selecting a single row in the same table/view if a query returns no results - sql

Question

I have the following view in my SQL database, which selects data from a Transaction table and a Customer table:
+-------+-----------+---------------------+--------+
| RowNo | Name | Date | Amount |
+-------+-----------+---------------------+--------+
| 1 | Customer1 | 2018-11-10 01:00:00 | 55.49 |
| 2 | Customer2 | 2018-11-10 02:00:00 | 58.15 |
| 3 | Customer3 | 2018-11-10 03:00:00 | 79.15 |
| 4 | Customer1 | 2018-11-11 04:00:00 | 41.89 |
| 5 | Customer2 | 2018-11-11 05:00:00 | 5.15 |
| 6 | Customer3 | 2018-11-11 06:00:00 | 35.17 |
| 7 | Customer1 | 2018-11-12 07:00:00 | 43.78 |
| 8 | Customer1 | 2018-11-12 08:00:00 | 93.78 |
| 9 | Customer2 | 2018-11-12 09:00:00 | 80.74 |
+-------+-----------+---------------------+--------+
I need an SQL query that will return all a customer's transactions for a given day (easy enough), but then if a customer had no transactions on the given day, the query must return the customer's most recent transaction.
Edit:
The view is as follows:
Create view vwReport as
Select c.Name, t.Date, t.Amount
from Transaction t
inner join Customer c on c.Id = t.CustomerId
And then to get the data I just do a select from the view:
Select * from
vwReport r
where r.Date between '2018-11-10 00:00:00' and '2018-11-11 00:00:00'
So, to clarify, I need one query that returns all the customer transactions for a day, and included in that results set is the last transaction of any customers who don't have a transaction on that day. So, in the table above, running the query for 2018-11-12, should return row 7, 8 and 9, as well as row 6 for Customer3 that did not have a transaction on the 12th.

Answer 1

Take your existing query and UNION ALL it with a "most recent transaction query" for everyone who doesn't have a transaction in that range.
with found as
(
select c.Id, c.Name, t.Date, t.Amount
from Transaction t
inner join Customer c on c.Id = t.CustomerId
where Date between '2018-11-10 00:00:00' and '2018-11-11 00:00:00'
)
with unfound as
(
select c.Id, c.Name, t.Date, t.Amount, RANK() OVER (PARTITION BY Name ORDER BY CAST(Date AS DATE) DESC) AS row
from Transaction t
inner join Customer c on c.Id = t.CustomerId
WHERE Date < '2018-11-10 00:00:00'
)
select Name, Date, Amount
from found
union all
select Name, Date, Amount
from unfound
where Id not in ( select Id from found ) and row = 1

Answer 2

You're interested in selecting multiple rows with ties, you could use the RANK() function to find all rows ranked by date descending:
SELECT * FROM (
SELECT *, RANK() OVER (PARTITION BY Name ORDER BY CAST(Date AS DATE) DESC) AS rn
FROM txntbl
WHERE CAST(Date AS DATE) <= '2018-11-12'
) AS x
WHERE rn = 1
Demo on DB Fiddle

Answer 3

You can use a correlated subquery:
select t.*
from transactions t
where t.date = (select max(t2.date)
from transactions t2
where t2.name = t.name and
t2.date <= #date
);
Note: This only returns customers who had a transaction on or before the date in question.

Answer 4

With the limited information available from the question, the following presents a solution using a join as opposed to a correlated subquery:
select t1.*
from
vwReport t1 inner join
(
select t2.name, max(t2.date) as mdate
from vwReport t2
group by t2.name
) t3
on t1.name = t3.name and t1.date = t3.mdate
where
t1.date <= #date

Answer 5

Use UNION for the last date transactions only if there are no transactions for the given dates (BETWEEN '2018-11-10 00:00:00' AND '2018-11-11 00:00:00'):
SELECT * FROM vwReport r
WHERE (r.Date BETWEEN '2018-11-10 00:00:00' AND '2018-11-11 00:00:00')
AND (r.Name = #name)
UNION
SELECT * FROM vwReport r
WHERE (r.Date = (SELECT MAX(r.Date) FROM vwReport r WHERE r.Name = #name))
AND (r.Name = #name)
AND ((SELECT COUNT(*) FROM vwReport r
WHERE (r.Date BETWEEN '2018-11-10 00:00:00' AND '2018-11-11 00:00:00')
AND (r.Name = #name)) = 0)