fix a bug when using MySQL
fix a bug when using MySQL

cmu-db/cmdbac,cmu-db/cmdbac,cmu-db/db-webcrawler,cmu-db/cmdbac,cmu-db/db-webcrawler,cmu-db/cmdbac,cmu-db/db-webcrawler,cmu-db/db-webcrawler,cmu-db/db-webcrawler,cmu-db/cmdbac