Comparing to a null string in PL/SQL - oracle

Question

I am looking for a way to compare a string to a null string the optimal way.
If I do
IF(str1 <> str2) THEN
dbms_output.put_line('Strings are not equal');
END IF;
This gets ignored if str1 is NULL.
I know I can set a null string to something before comparing to make it work
str1 := NVL(str1, 'Empty');
IF(str1 <> str2) THEN
dbms_output.put_line('Strings are not equal');
END IF;
And that works but I wanted to see if there is a better way to handle null string comparisons

Answer 1

You didn't say whether the string you must compare (str1) may be null; nor what the result of the comparison should be if either string is null.
It very rarely (if ever) makes sense to consider a non-null string equal to null. Sometimes it does make sense to consider two null strings as equal (if you think of them as "empty strings" - idiotically, Oracle doesn't make the distinction, mandated by the SQL Standard, between the SQL concept of null, on the one hand, and the empty string concept on the other hand).
The condition below:
str1 = str2
returns TRUE if both strings are non-null and they are equal; it returns FALSE if both strings are non-null and they are different; and it returns UNKNOWN if at least one string is null.
The LNNVL operator (which takes a condition as its argument) returns FALSE when its argument is TRUE and it returns TRUE when its argument is either FALSE or UNKNOWN. So, the condition
lnnvl(str1 = str2)
returns FALSE when the strings are both non-null and equal, and TRUE in all other cases. If you must consider two null strings as "not equal", then this is the condition you need in your code.
If, on the other hand, you must view two null strings as equal, you can use
decode(str1, str2, 0) is not null
In the case above, DECODE returns 0 if str1 and str2 are both non-null and equal, and also when they are both null. (This is why DECODE is often used in exactly this kind of situation). It returns the default (which I didn't specify, so the function will return the "default" default value, which is NULL) in the remaining cases. DECODE will return non-NULL when str1 and str2 are "the same" - either as equal, non-null strings, or as both null.
Of course, the DECODE condition can also be written as
decode(str1, str2, 0) = 0
or even
decode(str1, str2, 0, 1) = 0
to make it more explicit. Some people (not me!) may prefer it that way.

Answer 2

What if str2 literally is 'Empty'? nvl(str1, 'Empty'); (which is what you mean by DECODE(str1, 'Empty') I assume (decode() needs at least three parameters so your original expression would fail)) will give you a false negative. Just use IS NULL.
IF str1 <> str2
OR str1 IS NULL
OR str2 IS NULL THEN
dbms_output.put_line('Strings are not equal');
END IF;

Answer 3

If either str1 or str2 can be NULL then you can use:
IF str1 <> str2
OR str1 IS NULL AND str2 IS NOT NULL
OR str1 IS NOT NULL AND str2 IS NULL
THEN
dbms_output.put_line('Strings are not equal');
END IF;
db<>fiddle here