Problem

The actual value of a custom field from an Issue should be selected

Solution

select CAST(pk.PROJECT_KEY as VARCHAR(10))+'-'+CAST(issue.issuenum AS varchar(10)), cfo.customvalue FROM 
CustomFieldValue cfv 
INNER JOIN CustomField CF on CF.Id = CFV.CustomField
INNER JOIN CustomFieldOption CFO on CF.Id  = CFO.CustomField
INNER JOIN jiraissue issue on issue.id = cfv.issue 
INNER JOIN project_key pk on PROJECT_ID=issue.PROJECT
WHERE cf.CFName = 'Classification' 
And CAST(CFO.Id AS VARCHAR(10)) =CFV.StringValue;