摘要： Three-Valued Logic(三值逻辑)SQL values: TRUE, FALSE, UNKNOWN由于查询筛选器中(ON, WHERE, and HAVING) 将UNKNOW认定为FALSE,所以代入后表达式为 (NULL=NULL,NULL>5, NULL<NULL)之类的行集都会被过滤掉而不出现在结果集中.UNKNOWN logical results and NULLs are treated inconsistently in different elements of the language:all query filters (ON, WHERE, an 阅读全文