Database: academic_minus_1_0
Question: return me the number of authors who have papers containing keyword " Relational Database " .
SQL: select count ( distinct t4.name ) from publication_keyword_keyword as t1 join publication as t2 on t1.pid = t2.pid join writes as t3 on t2.pid = t3.pid join author as t4 on t3.aid = t4.aid where t1.keyword = 'relational database' ;