SQL Server: combining multiple rows into one row

There are several methods.

If you want just the consolidated string value returned, this is a good quick and easy approach

DECLARE @combinedString VARCHAR(MAX)
SELECT @combinedString = COALESCE(@combinedString + ', ', '') + stringvalue
FROM jira.customfieldValue
WHERE customfield = 12534
    AND ISSUE = 19602

SELECT @combinedString as StringValue 

Which will return your combined string.

You can also try one of the XML methods e.g.

SELECT DISTINCT Issue, Customfield, StringValues
FROM Jira.customfieldvalue v1
CROSS APPLY ( SELECT StringValues + ',' 
              FROM jira.customfieldvalue v2
              WHERE v2.Customfield = v1.Customfield 
                  AND v2.Issue = v1.issue 
              ORDER BY ID 
                  FOR XML PATH('') )  D ( StringValues )
WHERE customfield = 12534
    AND ISSUE = 19602

You can achieve this is to combine For XML Path and STUFF as follows:

SELECT (STUFF((
        SELECT ', ' + StringValue
        FROM Jira.customfieldvalue
        WHERE CUSTOMFIELD = 12534
        AND ISSUE = 19602
        FOR XML PATH('')
        ), 1, 2, '')
    ) AS StringValue