I am using Oracle DB. At the database level, when you set a column value to either NULL or '' (empty string), the fetched value is NULL in both cases. Is it possible to store '' (empty string) as a non NULL value in the database?
I execute this
UPDATE contacts SET last_name = '' WHERE id = '1001';
commit;
SELECT last_name, ID FROM contacts WHERE id ='1001';
LAST_NAME ID
------------ ------
null 1001
Is it possible to store the last_name as a non-NULL empty string ('')?
The only way to do this in oracle is with some kind of auxiliary flag field, that when set is supposed to represent the fact that the value should be an empty string.
As far as i know Oracle does not distinguish between '' and NULL, see here.
Oracle has a well know behavior that it silently converts "" to NULL on INSERT and UPDATE statements.
You have to deal with this in your code to prevent this behavior by converting NULL to "" when you read the columns back in and just do not use null in your program to begin with.
A long time since I used Oracle, but I believe we used to use a single space ' ' to represent an empty string, then trim it after reading.
If you use a VARCHAR2 data type then NULL and '' are identical and you cannot distinguish between them; so, as mentioned in other answers, you would either need to:
Have an additional column that contains a flag that distinguishes between non-NULL and NULL values so that if then flag states it is non-NULL and it contains a NULL then you know it is an empty string; or
Use an alternate representation, such as a single space character, for an empty string. This would then mean that you cannot store a string with that alternate representation; however, if trailing white-space was syntactically invalid for the strings you are storing then using a single space character to represent an empty string would be fine.
If you are using a CLOB data type then you CAN store an empty string using the EMPTY_CLOB() function:
CREATE TABLE table_name (value CLOB);
INSERT INTO table_name (value) VALUES (NULL);
INSERT INTO table_name (value) VALUES (EMPTY_CLOB());
INSERT INTO table_name (value) VALUES ('A');
Then:
SELECT value, LENGTH(value) FROM table_name;
Outputs:
VALUE
LENGTH(VALUE)
null
null
0
A
1
db<>fiddle here
Related
I have a case statement to rectify one business logic in snowflake:
INSERT INTO DB.table_b
SELECT
CASE
WHEN UPPER(emp) <> LOWER(emp) THEN NULL
WHEN emp IS NULL THEN nullif(emp, 'NULL')
ELSE emp
END AS emp_no
FROM
DB.table_a;
The 'table_a' content as below :
emp
-------
ABCD
NULL
''
23
It contains character string, null, empty and numbers. So, the requirement is to take only numbers and empty values from the case statement since the column emp_no in 'table_b' is numeric type. In source table if the column value is string then we have to insert NULL value. But as the 'table_b' column is of type 'numeric' the null value is not getting inserted and getting following error
Numeric value '' is not recognized
Using TRY_TO_NUMBER:
A special version of TO_DECIMAL , TO_NUMBER , TO_NUMERIC that performs the same operation (i.e. converts an input expression to a fixed-point number), but with error-handling support (i.e. if the conversion cannot be performed, it returns a NULL value instead of raising an error).
INSERT INTO DB.table_b
SELECT TRY_TO_NUMBER(emp) AS emp
FROM DB.table_a;
you can not use IS_INTEGER but for VARCHAR(16777216) it isn't supported
So a regular expression would be better
INSERT INTO DB.table_b
SELECT
CASE
WHEN regexp_like(emp,'^[0-9]+$') THEN emp
ELSE NULL
END AS emp_no
FROM
DB.table_a;
As Lukasz mentions you should use the TRY_TO_x functions (TRY_TO_NUMERIC, TRY_TO_DOUBLE) as these safely handle parsing the types, and return NULL if the parse fails. The extra note I will add is that both NUMBER/NUMERICs and DOUBLEs will parse 0.1234 but get different results, which you didn't mention as caring about, but I think is worth noting, so I am adding an extra answer to point the difference out.
The CTE is just to get the values into the SQL:
WITH data(emp) as (
select * from values
('ABCD'),
(NULL),
(''),
('0.123'),
('23')
)
SELECT emp
,try_to_numeric(emp) as emp_as_num
,try_to_double(emp) as emp_as_float
FROM data
EMP
EMP_AS_NUM
EMP_AS_FLOAT
'ABCD'
null
null
null
null
null
''
null
null
'0.123'
0
0.123
'23'
23
23
You can test for amp being string and set the string to NULL. Only numeric values will go into the second case statement.
SELECT
CASE
WHEN IS_VARCHAR(emp) then NULL else
case WHEN UPPER(emp) <> LOWER(emp) THEN NULL ELSE emp end
end AS emp_no
I am reading a flat file in hive and i have null values coming in file like below
a|b|null|null|d
and when I create table on top of this with below datatypes
a char(1),b char(1),c char(1),varchar2(2),char(1)
and the value in table coming like this
a,b,n,nu,d
The oneway I can do this is to make the datatype as varchar2(4) and add check at null.
But is there any other way i can do this.
SerDe treats 'null' strings as normal values, no difference between value 'a' and 'null'.
Try to add 'serialization.null.format'='null' property to your table definition:
ALTER TABLE mytable SET tblproperties('serialization.null.format'='null');
Another approach is to use STRING data type and case statements is select:
select case when col = 'null' then null end as col
...
I try to use stored procedure in my project.However i have question about null and empty.
ALTER PROCEDURE [dbo].[SP_EMAIL_LIST]
AS
BEGIN
SELECT *
FROM CUSTOMER
WHERE
EMAIL_ADRESS IS NOT NULL
END
I used where clause for avoid getting values null or empty values however i still get null values.
How can i get email adress values if not null and not empty ?
This should work.
SELECT
*
FROM
CUSTOMER
WHERE
EMAIL_ADRESS IS NOT NULL AND EMAIL_ADRESS != ''
Keep in mind there is a difference between NULL and ''. '' is simply an empty string. NULL generally is used to indicate an unknown or unspecified value.
Primarily as an SQL Server user I was suprised that in Oracle this syntax is valid:
select var
from table
where substr(var, 2, 1) is null
How can a subsrting from varchar string variable be null? In SQL Server this would never be true(?).
select var
from table
where substring(var, 2, 1) is null
In many cases in Oracle, the empty string is actually NULL.
If you substr() outside the range of your in-string, SQL server returns the empty string, while Oracle returns NULL (its equivalent to the empty string).
In general, null means absence of value.
select substr ('abcd',2, 1) from dual;
-- result: 'b'
select substr ('a',2, 1) from dual;
-- empty, because there is nothing at second position on string 'a';
select decode(substr('a',2,1),null, 'yes, is null') from dual;
-- result: 'yes, is null'
select decode('', null, 'yes, is null') from dual;
-- result: 'yes, is null'
Update:
It is clear a choice of implementation.
In most languages, for example in Java, Strings are placed in a memory area, and, for programmer, string variables are references to the area in the memory. So, an empty string is a reference to an area of length 0, but the reference isn't null. So, null is a different thing(no reference) and I think like you: it is better.
I am using Oracle DB. At the database level, when you set a column value to either NULL or '' (empty string), the fetched value is NULL in both cases. Is it possible to store '' (empty string) as a non NULL value in the database?
I execute this
UPDATE contacts SET last_name = '' WHERE id = '1001';
commit;
SELECT last_name, ID FROM contacts WHERE id ='1001';
LAST_NAME ID
------------ ------
null 1001
Is it possible to store the last_name as a non-NULL empty string ('')?
The only way to do this in oracle is with some kind of auxiliary flag field, that when set is supposed to represent the fact that the value should be an empty string.
As far as i know Oracle does not distinguish between '' and NULL, see here.
Oracle has a well know behavior that it silently converts "" to NULL on INSERT and UPDATE statements.
You have to deal with this in your code to prevent this behavior by converting NULL to "" when you read the columns back in and just do not use null in your program to begin with.
A long time since I used Oracle, but I believe we used to use a single space ' ' to represent an empty string, then trim it after reading.
If you use a VARCHAR2 data type then NULL and '' are identical and you cannot distinguish between them; so, as mentioned in other answers, you would either need to:
Have an additional column that contains a flag that distinguishes between non-NULL and NULL values so that if then flag states it is non-NULL and it contains a NULL then you know it is an empty string; or
Use an alternate representation, such as a single space character, for an empty string. This would then mean that you cannot store a string with that alternate representation; however, if trailing white-space was syntactically invalid for the strings you are storing then using a single space character to represent an empty string would be fine.
If you are using a CLOB data type then you CAN store an empty string using the EMPTY_CLOB() function:
CREATE TABLE table_name (value CLOB);
INSERT INTO table_name (value) VALUES (NULL);
INSERT INTO table_name (value) VALUES (EMPTY_CLOB());
INSERT INTO table_name (value) VALUES ('A');
Then:
SELECT value, LENGTH(value) FROM table_name;
Outputs:
VALUE
LENGTH(VALUE)
null
null
0
A
1
db<>fiddle here