DECLARE
str1 varchar2(4000);
str2 varchar2(4000);
BEGIN
str1:='';
str2:='sdd';
IF(str1<>str2) THEN
dbms_output.put_line('The two strings is not equal');
END IF;
END;
/
这很明显，str1和str2两个字符串不相等，但为什么“两个字符串不相等”没有打印出来？ Oracle有另外一种常用的方法来比较两个字符串？
DECLARE
str1 varchar2(4000);
str2 varchar2(4000);
BEGIN
str1:='';
str2:='sdd';
-- Provide an alternate null value that does not exist in your data:
IF(NVL(str1,'X') != NVL(str2,'Y')) THEN
dbms_output.put_line('The two strings are not equal');
END IF;
END;
/
关于空比较：
根据Oracle 12c documentation on NULLS，使用IS NULL或IS NOT NULL的空比较会计算为TRUE或FALSE。然而，所有其他比较评估为UNKNOWN，而不是FALSE。文件进一步说明：
A condition that evaluates to UNKNOWN acts almost like FALSE. For example,a SELECT statement with a condition in the WHERE clause that evaluates to UNKNOWN returns no rows. However,a condition evaluating to UNKNOWN differs from FALSE in that further operations on an UNKNOWN condition evaluation will evaluate to UNKNOWN. Thus,NOT FALSE evaluates to TRUE,but NOT UNKNOWN evaluates to UNKNOWN.
Oracle提供了一个参考表：
Condition Value of A Evaluation ---------------------------------------- a IS NULL 10 FALSE a IS NOT NULL 10 TRUE a IS NULL NULL TRUE a IS NOT NULL NULL FALSE a = NULL 10 UNKNOWN a != NULL 10 UNKNOWN a = NULL NULL UNKNOWN a != NULL NULL UNKNOWN a = 10 NULL UNKNOWN a != 10 NULL UNKNOWN
我也知道，我们不应该写PL / sql，假设空字符串总是会被计算为NULL：
Oracle Database currently treats a character value with a length of zero as null. However,this may not continue to be true in future releases,and Oracle recommends that you do not treat empty strings the same as nulls.
