Problem
A string, date or a text-value of an custom field shall be retrieved from an issue
Solution
SELECT pk.PROJECT_KEY+'-'+CAST(ji.issuenum AS varchar(max)) as issue,
cfv.STRINGVALUE,
cfv.TEXTVALUE,
cfv.DATEVALUE
FROM jiraissue ji
INNER JOIN customfieldvalue cfv ON ji.id=cfv.issue
INNER JOIN project_key pk ON ji.PROJECT=pk.PROJECT_ID
WHERE pk.PROJECT_KEY+'-'+CAST(ji.issuenum AS varchar(max))='AT-15'
AND cfv.CUSTOMFIELD=(SELECT TOP 1 id FROM customfield WHERE cfname='Requested document')