Oracle empty string converts to null

Question

When doing an insert, an empty string is converted to null:

insert into test (f) values ('');

Now, there is a row with f containing a null.

But, when I query the table, I cannot use '':

select * from test where f='';
no rows selected

I can use null:

select * from test where f is null;
____F_
NULL

So... it appears that Oracle decided that empty strings cannot be used for insert, but they remain empty strings when doing queries. Where is the documentation on when an empty string becomes a null and when it remains an empty string?

Answer 1

This says it all:

select NVL('','it is null') as value
from dual;

SQL Fiddle

2 things:

1) '' gets converted to NULL on insert. That's an Oracle VARCHAR2 thing.

2) select * from test where f=''; is trying to do select * from test where f=NULL, which isn't defined, and will return nothing because NULL doesn't like the equality operator. You have to use IS NULL or IS NOT NULL.

I'll add that the CHAR datatype behaves differently because it is padded.

Answer 2

Oracle treats '' and NULL the same. When inserting '', there is no conversion of '' to NULL, merely an interpretation of '' as NULL in the same way that the word NULL is interpreted as NULL or rtrim('a','a') is interpreted as NULL.

Here is a demonstration using the following table and insert:

drop table t1;
create table t1 (c1 varchar2(10));
insert into t1 (c1) values ('');

The insert above inserted a NULL value for c1. You can select that row as follows:

SELECT c1 FROM t1;

When you add a WHERE clause to compare equality and one of the values being compared is NULL, the result will always be unknown. Unknown will evaluate to false except that further operations on an unknown value produce unknown values. All of the following return no rows because the WHERE clauses contain conditions that will never be true regardless of the data.

SELECT c1 FROM t1 WHERE c1 = '';
SELECT c1 FROM t1 WHERE c1 = NULL;
SELECT c1 FROM t1 WHERE '' = '';
SELECT c1 FROM t1 WHERE NULL = NULL;

Oracle provides a special syntax to retrieve rows with a particular column having null values -- IS NULL.

SELECT c1 FROM t1 WHERE c1 IS NULL;

There are a few conditions in which oracle compares NULLS treating them as equal to other NULL values such as in DECODE statements and in compound keys.

More information can be found in the SQL Language Reference.

Answer 3

Oracle RDBMS is not making any differences between null and empty strings; meaning that there is no transformation to convert an empty string into a null value.

I can be understood that none of the 2 is containing information.

Answer 4

This explains it a little:

create table t1 (c1 char(1),
                 c2 char(3),
                 c3 varchar2(10));


insert into t1 values ('','','');
insert into t1 values ('a','a','a');

declare 
v1 char(1);
begin
v1 := '';
insert into t1 values (v1,v1,v1);
end;

select * from t1;

select count(c1) from t1;
select count(c2) from t1;
select count(c3) from t1;

select length(c1), length(c2), length(c3) from t1;