Database: academic
Question: return me the number of authors who have papers containing keyword " Relational Database " .
SQL:
SELECT COUNT ( DISTINCT t2.name ) FROM publication_keyword AS t5 JOIN keyword AS t1 ON t5.kid  =  t1.kid JOIN publication AS t3 ON t3.pid  =  t5.pid JOIN writes AS t4 ON t4.pid  =  t3.pid JOIN author AS t2 ON t4.aid  =  t2.aid WHERE t1.keyword  =  "Relational Database";