By Chris R


2008-10-15 02:04:18 8 Comments

I know that it does consider ' ' as NULL, but that doesn't do much to tell me why this is the case. As I understand the SQL specifications, ' ' is not the same as NULL -- one is a valid datum, and the other is indicating the absence of that same information.

Feel free to speculate, but please indicate if that's the case. If there's anyone from Oracle who can comment on it, that'd be fantastic!

10 comments

@zloctb 2015-08-08 15:11:37

Example from book

   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;

@Sorter 2015-03-23 11:34:49

According to official 11g docs

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.

Possible reasons

  1. val IS NOT NULL is more readable than val != ''
  2. No need to check both conditions val != '' and val IS NOT NULL

@ErikE 2016-02-03 16:02:34

In a fully ANSI-compliant database, you don't have to check for both conditions. val <> '' already excludes NULL. Perhaps you meant val = '' OR val IS NULL. But empty strings that don't compare as NULL are useful!

@Alex Kreutznaer 2013-06-05 16:33:04

Empty string is the same as NULL simply because its the "lesser evil" when compared to the situation when the two (empty string and null) are not the same.

In languages where NULL and empty String are not the same, one has to always check both conditions.

@Egor Skriptunoff 2013-06-29 23:27:17

Simply set not null constraint on your column and check only on empty string.

@user565869 2014-07-09 14:24:55

Checking both conditions is trivial: WHERE Field <> '' returns true only if the field is not NULL and not empty, on databases with ANSI behavior for empty strings.

@Michael T. Gunderson 2011-07-14 02:27:56

First of all, null and null string were not always treated as the same by Oracle. A null string is, by definition, a string containing no characters. This is not at all the same as a null. NULL is, by definition, the absence of data.

Five or six years or so ago, null string was treated differently from null by Oracle. While, like null, null string was equal to everything and different from everything (which I think is fine for null, but totally WRONG for null string), at least length(null string) would return 0, as it should since null string is a string of zero length.

Currently in Oracle, length(null) returns null which I guess is O.K., but length(null string) also returns null which is totally WRONG.

I do not understand why they decided to start treating these 2 distinct "values" the same. They mean different things and the programmer should have the capability of acting on each in different ways. The fact that they have changed their methodology tells me that they really don't have a clue as to how these values should be treated.

@user565869 2014-07-09 14:27:38

Citation required for making a distinction between "null string" and NULL value. In any database except Oracle, a VARCHAR field can have a value (zero or more characters) or no value (NULL), full stop.

@William Robertson 2018-07-21 10:34:51

"Five or six years ago" from 2011 would fall in the 10g timeframe (10.1 released 2003, 10.2 in 2005). 10g absolutely did not introduce any global changes in the handling of nulls, and there has never been any distinction between NULL and a null valued string, and such a distinction make no sense. I'm afraid this answer is a complete fantasy.

@Cade Roux 2008-10-15 02:27:46

Indeed, I have had nothing but difficulties in dealing with Oracle, including invalid datetime values (cannot be printed, converted or anything, just looked at with the DUMP() function) which are allowed to be inserted into the database, apparently through some buggy version of the client as a binary column! So much for protecting database integrity!

Oracle handling of NULLs links:

http://digitalbush.com/2007/10/27/oracle-9i-null-behavior/

http://jeffkemponoracle.com/2006/02/empty-string-andor-null.html

@Mark Brady 2008-10-15 15:17:26

invalid datatime values? Not sure what that means. Have you posted this as a question here?

@Cade Roux 2008-10-15 15:56:34

The problem pre-dated stackoverflow - I got no useful information from Oracle forums and I created a workaround - I'll track my notes down and post here.

@Cade Roux 2008-10-15 16:13:34

Posted details as a question here.

@erikkallen 2010-04-09 19:52:59

Because not treating it as NULL isn't particularly helpful, either.

If you make a mistake in this area on Oracle, you usually notice right away. In SQL server, however, it will appear to work, and the problem only appears when someone enters an empty string instead of NULL (perhaps from a .net client library, where null is different from "", but you usually treat them the same).

I'm not saying Oracle is right, but it seems to me that both ways are approximately equally bad.

@Lucio M. Tato 2013-10-18 20:02:12

Much, much more easier to debug. Also, if you see an empty cell or input on the screen, you know the data in the DB is null. In other DBs where ''<>NULL, you can't "see" if the data is null or '', this leads to very sneaky bugs. ''=null it's the sanest option, even if it is not standard.

@Didier L 2014-09-23 14:18:33

“In other DBs where ''<>NULL, you can't "see" if the data is null or ''” => Usually, DB tools display NULLs differently from empty strings. Actually, even Oracle SQL Developer shows NULLs as “(null)”. I guess this is to distinguish NULL from whitespace but it is unrelated to the difference between NULL and empty strings.

@Justin Cave 2008-10-15 02:30:04

I believe the answer is that Oracle is very, very old.

Back in the olden days before there was a SQL standard, Oracle made the design decision that empty strings in VARCHAR/VARCHAR2 columns were NULL and that there was only one sense of NULL (there are relational theorists that would differentiate between data that has never been prompted for, data where the answer exists but is not known by the user, data where there is no answer, etc. all of which constitute some sense of 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 has left open the possibility that the VARCHAR data type would change in a future release to adhere to the SQL standard (which is why everyone uses VARCHAR2 in Oracle since that data type's behavior is guaranteed to remain the same going forward).

@user67897 2009-02-18 15:50:41

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.

@Jared 2015-11-13 19:07:07

That only makes sense if you assume that every field in a data entry system is mandatory. A non-answer to a non-mandatory field (e.g. "Dog's Name") is valid, so an empty string still has a distinct purpose from NULL. Even with that assumption in place, I doubt early developers thought of Oracle as a "glorified backend for a data entry system" so I'm not sure this answer makes sense at all.

@Walter Mitty 2008-10-15 13:46:13

Oracle documentation alerts developers to this problem, going back at least as far as version 7.

Oracle chose to represent NULLS by the "impossible value" technique. For example, a NULL in a numeric location will be stored as "minus zero", an impossible value. Any minus zeroes that result from computations will be converted to positive zero before being stored.

Oracle also chose, erroneously, to consider the VARCHAR string of length zero (the empty string) to be an impossible value, and a suitable choice for representing NULL. It turns out that the empty string is far from an impossible value. It's even the identity under the operation of string concatenation!

Oracle documentation warns database designers and developers that some future version of Oracle might break this association between the empty string and NULL, and break any code that depends on that association.

There are techniques to flag NULLS other than impossible values, but Oracle didn't use them.

(I'm using the word "location" above to mean the intersection of a row and a column.)

@Piotr Dobrogost 2018-10-15 20:44:11

Oracle documentation warns database designers and developers that some future version of Oracle might break this association between the empty string and NULL, and break any code that depends on that association – could you please provide reference for this statement?

@Walter Mitty 2018-10-16 00:29:29

@Brian 2008-10-15 02:38:52

Tom Kyte VP of Oracle:

A ZERO length varchar is treated as NULL.

'' is not treated as NULL.

'' when assigned to a char(1) becomes ' ' (char types are blank padded strings).

'' when assigned to a varchar2(1) becomes '' which is a zero length string and a zero length string is NULL in Oracle (it is no long '')

@Chris R 2008-10-15 03:10:38

Wow, Tom's pretty snarky. Given that the questions are pertaining to an egregious divergence from SQL92, you'd think he'd be less punchy about it... although he might be tired of answering.

@Chris Gill 2009-08-27 12:17:45

The best thing about Tom is that you get a clear answer, which states exactly what he thinks. Look for some of the comments where people have used text speak on Ask Tom

@ypercubeᵀᴹ 2011-07-05 07:48:15

But it would be more precise if the second line was changed to '' is not always treated as NULL.

@sehe 2013-07-19 15:17:09

@ypercube The quote does not get more precise by changing the word actually used by Tom. If you think Tom worded it confusingly, mmm. Maybe. I think he's spot on. The most confusing situations arise when '' is being implicitely converted to a VARCHAR2, such as cast('' as char(1)) is null which is... surprisingly TRUE

@matt freake 2013-08-22 15:58:46

@sehe the confusing bit for me is select 1 from dual where ('' is null)

@Patrick Fromberg 2014-03-28 18:29:41

I expect '' not to be treated as NULL literal strings are chars no varchars. Logic. But why then does the following query return zero rows? select 1 as one from dual where 'a' > '' or 'a' < ''

@Falco 2014-05-06 15:24:48

Because any comparision with a NULL value returns neither TRUE nor FALSE but UNKNOWN... and UNKNOWN OR UNKNOWN is UNKNOWN and will never justify a where-condition. You can easily check this, by surrounding your whole where-condition with NOT(...) and it will still yield 0 rows ;-)

@Brian 2014-05-06 18:59:47

@Falco the discussion is around '' being treated as a null. Not the behavior of a null value.

@Falco 2014-05-07 15:44:09

But if Patrick expects '' to be treated as NULL, why is he surprised it shows the SAME behaviour like an actual NULL value? The outcome of his line is the same whether he puts '' or NULL there

@Roland 2015-05-27 16:51:09

'' is not treated as NULL. Not correct, on my Oracle 11.2 the following code prints null: if '' is null then SYS.DBMS_OUTPUT.PUT_LINE('null');end if;

@William Robertson 2016-12-07 15:41:15

@Roland - in the context, he was saying '' is not treated as null in every situation. The next line gave an example where assigning it to a char triggered its blank padding behaviour (although chars can also be null). I'm guessing the PL/SQL compiler is following varchar2 semantics when it encounters an untyped ''.

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