Если проверено только одно значение req.reqCreatedBy
, может быть простой оператор IF, используйте CASE для нескольких проверенных значений.
SELECT req.reqID as Id,
reqDesc.titleText as Title,
req.reqCity as City,
req.reqState as State,
req.areaID as Area,
area.areaname,
reqType.typeTitle as Type,
req.reqCreatedDate as Created,
req.reqEndDate as `End`,
-- req.reqCreatedBy is '0' OR ''?
IF(req.reqCreatedBy = '0', 'Unknown', CONCAT(users.firstname, ' ', users.lastname)) AS Recruiter
FROM apps_job_request as req
INNER JOIN apps_job_request_description as reqDesc
ON req.reqTitle = reqDesc.titleID
INNER JOIN apps_job_request_type as reqType
ON reqDesc.typeID = reqType.typeID
INNER JOIN `assemble_users`.area AS area
ON area.areaid = req.areaID
INNER JOIN `assemble_users`.users AS users
ON users.username = req.reqCreatedBy
WHERE req.reqID is not null
AND req.reqActive = '1'