return me the total citations of all the papers in the VLDB conference .
SELECT SUM ( t2.citation_num ) FROM publication AS t2 JOIN conference AS t1 ON t2.cid  =  t1.cid WHERE t1.name  =  "VLDB";
SELECT SUM ( t2.citation_num ) FROM publication AS t2 JOIN conference AS t1 ON t2.cid  =  t1.cid WHERE t1.name  =  "VLDB";
[{'SUM ( t2.citation_num )': None}]