Hayes, Brian. Third Base(PDF). American Scientist (Sigma Xi, the Scientific Research Society). November–December 2001, 89 (6): 490–494 [2006-12-25]. (原始内容存档(PDF)于2005-12-31).
oracle.com
Lex de Haan and Gennick, Jonathan. Nulls: Nothing to Worry About. Oracle Magazine (Oracle). July–August 2005 [2007-04-30]. (原始内容存档于2006-11-02).
sqlservercentral.com
Coles, Michael. Null Versus Null?. SQL Server Central (Red Gate Software). February 26, 2007 [2007-04-30]. (原始内容存档于2007-04-25).
web.archive.org
Hayes, Brian. Third Base(PDF). American Scientist (Sigma Xi, the Scientific Research Society). November–December 2001, 89 (6): 490–494 [2006-12-25]. (原始内容存档(PDF)于2005-12-31).
Lex de Haan and Gennick, Jonathan. Nulls: Nothing to Worry About. Oracle Magazine (Oracle). July–August 2005 [2007-04-30]. (原始内容存档于2006-11-02).
Coles, Michael. Null Versus Null?. SQL Server Central (Red Gate Software). February 26, 2007 [2007-04-30]. (原始内容存档于2007-04-25).