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示例数据库:-- Sample Data for this Chapter-- Listing 4-1: Creation Script for Sample Database and TablesSET NOCOUNT ON;USE master;IF DB_ID('Performance') IS NULL CREATE DATABASE Performance;GOUSE Performance;GO-- Creating and Populating the Nums Auxiliary TableSET NOCOUNT ON;IF OBJECT_ID('dbo 阅读全文
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