If you use SQL Server 2016+, you may try to use a JSON-based approach. You need to transform the data into a valid JSON array and parse the JSON array with OPENJSON(). Note that STRING_SPLIT() is not an option here, because as is mentioned in the documentation, the output rows might be in any order and the order is not guaranteed to match the order of the substrings in the input string.
Table:
CREATE TABLE Data (
ID int,
[String] varchar(100)
)
INSERT INTO Data
(ID, [String])
VALUES
(546, '1,2,1,5,7,8'),
(486, '2,4,8,1,5,1'),
(465, '18,11,20,1,4,18,11'),
(484, '11,10,11,12,50,11')
Statement:
SELECT
ID,
CONCAT(FirstValue, ',', SecondValue) AS [String]
FROM (
SELECT
d.ID,
j.[value] As FirstValue,
LEAD(j.[value]) OVER (PARTITION BY d.ID ORDER BY CONVERT(int, j.[key])) AS SecondValue
FROM Data d
CROSS APPLY OPENJSON(CONCAT('[', d.[String], ']')) j
) t
WHERE t.FirstValue = '1'
Result:
----------
ID String
----------
465 1,4
486 1,5
486 1,
546 1,2
546 1,5
486 | 1,5,1?