SQL Server 2022

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CREATE TABLE Reps ([RepName] varchar(5), [RepId] int);
INSERT INTO Reps ([RepName], [RepId]) VALUES
    ('Fred', 25), ('Doris', 50), ('Sybil', 99);
3 rows affected

CREATE TABLE Customers ([CustomerId] int, [RepId] int);
INSERT INTO Customers ([CustomerId]) VALUES
    (13), (17), (51), (76), (87), (97), (123), (131), (285), (315), (381), (433), (464), (466), (536), (539), (549), (604), (681), (729), (792);
21 rows affected

WITH 
  cte_reps AS (SELECT RepId, ROW_NUMBER() OVER (ORDER BY RepId) - 1 rn, COUNT(*) OVER () cnt FROM Reps),
  cte_customers AS (SELECT *, ROW_NUMBER() OVER (ORDER BY CustomerId) - 1 rn FROM Customers)
UPDATE c
SET c.RepId = r.RepId
FROM cte_customers c INNER JOIN cte_reps r
ON r.rn = c.rn % r.cnt; 
21 rows affected

SELECT * FROM Customers;


CustomerId
RepId


13
25

17
50

51
99

76
25

87
50

97
99

123
25

131
50

285
99

315
25

381
50

433
99

464
25

466
50

536
99

539
25

549
50

604
99

681
25

729
50

792
99

CustomerId	RepId
13	25
17	50
51	99
76	25
87	50
97	99
123	25
131	50
285	99
315	25
381	50
433	99
464	25
466	50
536	99
539	25
549	50
604	99
681	25
729	50
792	99

CustomerId	RepId
13	25
17	50
51	99
76	25
87	50
97	99
123	25
131	50
285	99
315	25
381	50
433	99
464	25
466	50
536	99
539	25
549	50
604	99
681	25
729	50
792	99

CustomerId	RepId
13	25
17	50
51	99
76	25
87	50
97	99
123	25
131	50
285	99
315	25
381	50
433	99
464	25
466	50
536	99
539	25
549	50
604	99
681	25
729	50
792	99