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";