query same table simultaneously with mysql (See related posts)

USE reference;

SELECT c2.pqid AS candidate,c1.surname,c1.atitle,c1.title,c1.type,c1.journal,c1.pqid,c1.volume, c1.issn,c1.issue,c1.spage,c1.year
FROM citations AS c1, citations AS c2
WHERE c1.issn = c2.issn
AND c1.volume = c2.volume
AND c1.issue = c2.issue
AND c1.year = c2.year
AND c1.spage = c2.spage
AND c1.pqid != c2.pqid
AND c1.pqid = ''
;