主题：[求助] 空值判断

 'cmd.CommandText = "U  PDATE 流水账查询 SET 科目表_三级 = '" & st & "' WHERE 科目表_一级 = '" & km_yiji & "' and 科目表_二级 = '" & km_erji & "' and 科目表_三级 = '" & km_sanji & "' and 科目表_四级 = '" & km_siji & "' and 科目表_五级 = '" & km_wuji & "' and 科目表_收支类别 = '支出'"

红色部分变量为空值时，代码没有执行，下面这句可以执行，代码要怎么改

        
         cmd.CommandText = "U  PDATE 流水账查询 SET 科目表_三级 = '" & st & "' WHERE 科目表_一级 = '" & km_yiji & "' and 科目表_二级 = '" & km_erji & "' and 科目表_三级 = '" & km_sanji & "' and 科目表_收支类别 = '支出'"