An empty string is a string of length zero ( &quot; &quot; ). Normally, a value NULL is not the same as an empty string. In general, a term NULL is used to denote an undefined value, an unknown value, or &quot; no value has been entered yet. &quot;
空字符串是一个长度为零的字符串 ()。通常情况下，NULL 值与空字符串不一样。一般而言，术语 NULL 用于表示一个未定义的值、一个未知的值，或“尚未输入任何值”。