摘要： Oracle中NULL的用法:参考网址:http://www.oratechinfo.co.uk/nulls.html#null_defUsing NULLsDefinition of NULLUsing NULL in a mathematical expression always returns NULLAppending NULL to a non-null string does not affect the stringAggregate functions tend to "ignore" NULLsGROUP BY treats NULL as a sepe 阅读全文