SQL Server 2017

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create table Customers (custid int, Quantity float, Price float, ReportDate date);
insert into Customers (custid, Quantity, Price, ReportDate) values
     (371965, 12, 2, '9/1/2016'),
     (371965, 2, 5, '2/25/2018'),
     (377958, 45, 3, '9/1/2016'),
     (270723, 12, 1.25, '5/1/2014'),
     (270723, 10.86, 1.25, '6/1/2014'),
     (270723, 12.29, 1.3, '7/1/2014'),
     (270723, 12.29, 1.4, '9/15/2016');
    
7 rows affected

create table Discounts (id int, startdate date, discount float);
insert into discounts (id, startdate, discount) values
    (  1, '7/18/2013',      0.1),
    (  2, '1/10/2014',     0.25),
    (  3, '7/11/2016',     0.11),
    (  4, '9/14/2016',     0.12),
    (  5, '1/12/2017',     0.15),
    (  6, '2/6/2017',      0.22),
    (  7, '6/28/2017',     0.09);
7 rows affected

select c.*, d.*
from customers c inner join discounts d
on d.startdate = (select max(startdate) from discounts where startdate < c.reportdate)


custid
Quantity
Price
ReportDate
id
startdate
discount


371965
12
2
2016-09-01
3
2016-07-11
0.11

371965
2
5
2018-02-25
7
2017-06-28
0.09

377958
45
3
2016-09-01
3
2016-07-11
0.11

270723
12
1.25
2014-05-01
2
2014-01-10
0.25

270723
10.86
1.25
2014-06-01
2
2014-01-10
0.25

270723
12.29
1.3
2014-07-01
2
2014-01-10
0.25

270723
12.29
1.4
2016-09-15
4
2016-09-14
0.12

custid	Quantity	Price	ReportDate	id	startdate	discount
371965	12	2	2016-09-01	3	2016-07-11	0.11
371965	2	5	2018-02-25	7	2017-06-28	0.09
377958	45	3	2016-09-01	3	2016-07-11	0.11
270723	12	1.25	2014-05-01	2	2014-01-10	0.25
270723	10.86	1.25	2014-06-01	2	2014-01-10	0.25
270723	12.29	1.3	2014-07-01	2	2014-01-10	0.25
270723	12.29	1.4	2016-09-15	4	2016-09-14	0.12