SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE customers (
  id int NOT NULL,
  company varchar(255),
  first_name varchar(255) default NULL,
  last_name varchar(255) default NULL,
  PRIMARY KEY (id)
);

CREATE TABLE transactions (
  id int NOT NULL,
  form_type varchar(255) default NULL,
  customer_id int default NULL,
  due decimal default NULL,
  PRIMARY KEY (id)
);

INSERT INTO customers (id, company, first_name, last_name) VALUES
(1, 'MDD', 'T', 'H'),
(2, 'Aliance Magnet A LLP', 'A', 'Wilkinson'),
(3, 'MAF', 'C', 'G'),
(4, 'QL', 'F', 'B'),
(5, 'ARL', 'S', 'P'),
(6, 'Q Corp.', 'H', 'H'),
(7, 'VQDA', 'L', 'W'),
(8, 'AESC', 'E', 'W'),
(9, 'Placement Incorporated', 'C', 'Mendez'),
(10, 'Numpties United', 'Y', 'Cunningham');

INSERT INTO transactions (id, form_type, customer_id, due) VALUES
(1, 'invoice', 9, 1.08),
(2, 'payment', 1, 6.32),
(3, 'invoice', 7, 9.42),
(4, 'payment', 9, 3.58),
(5, 'invoice', 7, 5.35),
(6, 'payment', 3, 5.42),
(7, 'invoice', 9, 5.32),
(8, 'invoice', 9, 9.62),
(9, 'invoice', 10, 1.40),
(10, 'invoice', 2, 3.72);
20 rows affected

SELECT
  t.id AS trans_id,
  c.id AS customer_id,
  c.company,
  c.first_name,
  c.last_name,
  t.due
FROM (
  SELECT DISTINCT TOP 3  c.*
  FROM customers AS c
  JOIN transactions AS t ON t.customer_id = c.id
  WHERE t.due > 0
  AND t.form_type = 'invoice'
  ORDER BY c.company, c.first_name, c.last_name

) AS c
JOIN transactions AS t ON t.customer_id = c.id
WHERE t.due > 0 AND t.form_type = 'invoice'
ORDER BY c.company, c.first_name, c.last_name;


trans_id
customer_id
company
first_name
last_name
due


10
2
Aliance Magnet A LLP
A
Wilkinson
4

9
10
Numpties United
Y
Cunningham
1

1
9
Placement Incorporated
C
Mendez
1

7
9
Placement Incorporated
C
Mendez
5

8
9
Placement Incorporated
C
Mendez
10


SELECT
  t.id AS trans_id,
  c.id AS customer_id,
  c.company,
  c.first_name,
  c.last_name,
  t.due
FROM (
  SELECT DISTINCT TOP 3 c.*
  FROM customers AS c
  JOIN transactions AS t ON t.customer_id = c.id
  WHERE t.due > 0
  AND t.form_type = 'invoice'
  ORDER BY c.company, c.first_name, c.last_name
) AS c
JOIN transactions AS t ON t.customer_id = c.id
-- WHERE t.due > 0 AND t.form_type = 'invoice'
ORDER BY c.company, c.first_name, c.last_name;


trans_id
customer_id
company
first_name
last_name
due


10
2
Aliance Magnet A LLP
A
Wilkinson
4

9
10
Numpties United
Y
Cunningham
1

1
9
Placement Incorporated
C
Mendez
1

4
9
Placement Incorporated
C
Mendez
4

7
9
Placement Incorporated
C
Mendez
5

8
9
Placement Incorporated
C
Mendez
10

trans_id	customer_id	company	first_name	last_name	due
10	2	Aliance Magnet A LLP	A	Wilkinson	4
9	10	Numpties United	Y	Cunningham	1
1	9	Placement Incorporated	C	Mendez	1
7	9	Placement Incorporated	C	Mendez	5
8	9	Placement Incorporated	C	Mendez	10