Database: academic
Question: return me the number of authors who have papers in the VLDB conference .
SQL:
SELECT COUNT ( DISTINCT t1.name ) FROM publication AS t4 JOIN conference AS t2 ON t4.cid  =  t2.cid JOIN writes AS t3 ON t3.pid  =  t4.pid JOIN author AS t1 ON t3.aid  =  t1.aid WHERE t2.name  =  "VLDB";