因为不将其处理为NULL也不是特别有用。

如果你在Oracle上在这方面犯了错误，你通常会马上注意到。然而，在SQL server中，它似乎是可以工作的，并且只有当有人输入空字符串而不是NULL时才会出现问题(可能来自。net客户端库，其中NULL与“”不同，但您通常将它们视为相同的)。

我并不是说甲骨文是对的，但在我看来，这两种方法都差不多一样糟糕。

我相信答案是甲骨文非常非常古老。

早在SQL标准出现之前，Oracle的设计决定是VARCHAR/VARCHAR2列中的空字符串为NULL，并且只有一种意义上的NULL(有关系理论家会区分从未提示的数据、存在答案但用户不知道的数据、没有答案的数据等等，所有这些都构成了某种意义上的NULL)。

By the time that the SQL standard came around and agreed that NULL and the empty string were distinct entities, there were already Oracle users that had code that assumed the two were equivalent. So Oracle was basically left with the options of breaking existing code, violating the SQL standard, or introducing some sort of initialization parameter that would change the functionality of potentially large number of queries. Violating the SQL standard (IMHO) was the least disruptive of these three options.

Oracle保留了在未来的版本中更改VARCHAR数据类型以遵循SQL标准的可能性(这就是为什么每个人都在Oracle中使用VARCHAR2，因为数据类型的行为保证在未来保持不变)。

甲骨文公司副总裁汤姆·凯特:

长度为0的varchar被视为 NULL。 不被视为NULL。 当赋值给char(1)时变成 ' '(字符类型是空白填充的 字符串)。 当分配给varchar2(1)时 变成"，长度为零 字符串和零长度的字符串是 Oracle中的NULL(它不是长”)

根据官方的11g文件

Oracle数据库当前将长度为0的字符值视为null。然而，在未来的版本中可能不会继续如此，Oracle建议您不要将空字符串视为null。

可能的原因

val IS NOT NULL比val更可读!= " 不需要检查val != "和val IS NOT NULL这两个条件

I suspect this makes a lot more sense if you think of Oracle the way earlier developers probably did -- as a glorified backend for a data entry system. Every field in the database corresponded to a field in a form that a data entry operator saw on his screen. If the operator didn't type anything into a field, whether that's "birthdate" or "address" then the data for that field is "unknown". There's no way for an operator to indicate that someone's address is really an empty string, and that doesn't really make much sense anyways.

书中的例子

   set serveroutput on;   
    DECLARE
    empty_varchar2 VARCHAR2(10) := '';
    empty_char CHAR(10) := '';
    BEGIN
    IF empty_varchar2 IS NULL THEN
    DBMS_OUTPUT.PUT_LINE('empty_varchar2 is NULL');
    END IF;


    IF '' IS NULL THEN
    DBMS_OUTPUT.PUT_LINE(''''' is NULL');
    END IF;

    IF empty_char IS NULL THEN
    DBMS_OUTPUT.PUT_LINE('empty_char is NULL');
    ELSIF empty_char IS NOT NULL THEN
    DBMS_OUTPUT.PUT_LINE('empty_char is NOT NULL');
    END IF;

    END;