What is the datatype for NULL when passing that value for no data into a database?
There is NO data type of NULL. NULL itself means ABSENCE of data. When there is no data, how can it have type?
Null does not have a specific data type in SQL. Any nullable column or variable can contain null. Null is never equal or unequal to anything. You can cast a variable holding null to another variable and get null, for example:
declare #a integer
set #a = null
select convert (float, #a)
----------------------
NULL
(1 row(s) affected)
Usually NULL is its own datatype - the type of 1 is "INTEGER", the type of the type of NULL is "NULL"
Datatype for NULL is as meaningless as datatype for 0: it can be INTEGER, FLOAT or a VARCHAR. You cannot tell it just from the value.
NULL is legitimate value in almost every datatype domain, which means the absence of actual value.
It's also meaningless to discuss datatypes out of context of certain RDBMS.
In SQLite, for instance, datatypes are value-bound, not column-bound, and NULL is a first-class datatype per se.
In Oracle, the datatypes are more strictly defined. For instance, this query works:
SELECT COALESCE(dt, i)
FROM (
SELECT CAST(NULL AS DATE) AS dt, CAST(NULL AS DATE) i
FROM dual
) q
and this does not:
SELECT COALESCE(dt, i)
FROM (
SELECT CAST(NULL AS DATE) AS dt, CAST(NULL AS NUMBER) i
FROM dual
) q
, because the latter query returns two columns of different datatypes, both of them having values of NULL, and COALESCE requires both arguments to have same datatype.
It's better to say that a NULL of any datatype can be implicitly converted to a NULL on another datatype.
For instance, a VARCHAR can be implicitly converted to a INTEGER if it has value of 0, but cannot if it has value of 'some_string'.
For NULL's, any datatype can be implicitly converted to any other datatype, if the implicit conversion between them is allowed at all.
In SQL a NULL is a "mark" (something other than a value) that can apply to any SQL type. So it is orthogonal to type.
NULL is the value for 'undefined'. So any type in a database can be 'undefined', as it's a property of the column: a value of a row for the specific column can be 'undefined' which means it's 'NULL', no matter what the type is. As long as the column is nullable.
I think DBNULL or NULL is a special type.
I think the question defeats itself. If NULL had a datatype, wouldn't you be forced to change it with every instantiation outside of its default. For example, when you create it as a character, but then force it into an object's value?
NULL==NULL
That is all.
Actually, in PowerShell comparing $null -eq $null gives False.
Also, -not $null will give you True, so here it seems to be reprezented as False. I know, PowerShell might not be a good example, but still :)
Usually SQL NULL does not have a type associated with it. However there are exceptions. Database engines postgresql and derby (javadb) require that null has a type. In other words they do not support untyped null. So query conditions like NULL IS NULL may fail. Here, NULL must be given a type, the expected type of the target that processes the NULL value. In this case this appears silly because there is no target and this can be counterproductive.
See CAST function: -- you must cast NULL as a data type to use it
See Queries with guarded null Parameter fail and Add support for setObject(arg, null)
Please vote for these issues so that the odd database engines change their ways.
Prabhahar, each type of database driver has its own way of handling NULL. You will have to examine the driver API for the specific database.
For example if you are using the Java Derby database, simply pass in the Java native type, null as shown in Ian Bjorhovde's answer to "Derby's Handling of NULL Values":
insert into T_AUTHOR (
ID, FIRST_NAME, LAST_NAME,
DATE_OF_BIRTH, YEAR_OF_BIRTH, ADDRESS)
VALUES (
1000, 'Lukas', 'Eder',
'1981-07-10', null, null
);
Here is another null example of JDBC:Inserting null to Integer column:
pst.setNull(4, java.sql.Types.INTEGER);
http://en.wikipedia.org/wiki/Null_(SQL)
NULL can be cast (converted) to any data type yet data type comparisons with NULL always return FALSE.
Related
I try to use <select> to query int value in mysql with mybatis, it return null when nothing matches.
Howerver the corresponding method in XXXMapper.java return the primitive type int, which case exception: attempted to return null from a method with a primitive return type (int).
So i want to modify the default IntegerTypeHandler in mybatis, i want MyConfigIntegerTypeHandler returns -1 when nothing found.
I know the best way to solve this problem is to modify the method defination, to make the query method return Integer rather than int. I just want to know how to fix it without corrent the origin code.
I'm trying to make a mongo query (using Spring Data) for searching record by the Option value from Vavr. I have value Option userId and I'm wondering if it's possible to search in database by this parameter.
I have serched in MongoDB and Spring documentation, but in none of this is anything about Vavr type like Option. None of this query are working.
Option<Result> FindFirstByUserIdAndCreationDateDesc(String userId);
Option<Result> FindFirstByOptionUserIdAndCreationDateDesc(String userId);
where userId is Vavr Option
Option<String> userId;
Vavr's Option and Java's Optional are types for eliminating the null value.
Database don't use optional values, they use null values, so unwrap the value to a null.
userId.getOrElse(null)
However, be aware that in e.g. SQL databases, the null value doesn't compare equal to itself, so a WHERE userId = ? will return nothing if the ? argument is null. To find records with a null value, the SQL must be WHERE userId IS NULL, so unless your FindXxx methods knows this and account for it, the code will still not work for you.
i have a question.
I have a function called
fn_Sell(id integer, idCreditCard integer, description varchar)
thats an exameple, is not the all function, but i want to pass NULL, in idCreditCard, the field acepts null, but when i pass null value,
it said UNKONW kind of parameter (integer, UNKONW , varchar)
i think is cause of integer is not a null type of value.... what can i do?
y use callable statement in java, and for the second parameter i pass null
I think your issue is that you are passing in String "NULL" instead of NULL value. Both are different. Check what is passed through.
I have a database column whose type is "NUMBER" in oracle.
I want to map it by long or Long. But I'm not sure which type I should use, the primitive type or the Object type? Is there a convention? My case is that the value is the only thing I want from DAO method:
public class SampleDAO{
public long fetchNumberValue(){
//Is it better to return long value instead of Long?
}
}
And what if I want to wrap that value in an object?
public class SampleObject(){
//Is it better to use Long to use methods like hashCode() inherited from Object?
private Long value;
}
EDIT:
Thanks all for the answers. I got it that it depends on whether I need accept null value.
This database column is not nullable, actually it's the primary key. And it may have such values like 0, 1, 2 etc... OK, another question appears, how can I know if I get a matched row with a 0 value or no matched row? java.sql.ResultSet.getLong returns a long value and if the column's value is SQL NULL then 0 is returned. Or is it a bad practice to have a 0 valid in such a column?
Long will permit a null value, where as long must not be null. So I would match it do your database column - is it nullable or not?
Edit to the edited question:
To check if a value was actually 0 or null in the database, use ResultSet.wasNull.
It comes down to if you want to accept nulls or not.
Usually, you want to use the primitive type, and not the object, if the only thing you want is the value, and that value won't be null. For all your conversion needs, you have the class (such as String.parse).
You should consider also that Long objects consume more memory -not sure in what amount- than long primitives. If the Nullable attribute is not a problem, you should use primitive data types.
I'm querying a database and some of the results I'm getting are null. I'm setting these values to a variable with a datatype of double. Let's call the variable "results". So I tried setting up an if statement to see it equals Null which of course didn't work. Here is the code I have for that if statement:
if (results == null)
{
results = 0;
}
The error I get with this code is:
The operator == is undefined for the argument type(s) double, null
Is there a better way to determine if it's null?
Firstly, a Java double cannot be a Java null, and cannot be compared with null. (The double type is a primitive (non-reference) type and primitive types cannot be null.) I'm therefore assuming the following:
The "null" you are trying to detect is a NULL stored in the database that you are querying.
You are using JDBC to query the database.
If you call ResultSet.getDouble(...), that returns a double not a Double, the documented behavior is that a NULL (from the database) will be returned as zero. (See javadoc linked above.) That is no help if zero is a legitimate value for that column.
Changing the declared type of result to Double instead of double as suggested by Toby's answer will not help. When the database contains NULL, the value assigned to result will be a Double(0.0), not a null.
Here are two options that will work:
Use ResultSet.wasNull() to test for a (database) NULL ... immediately after the getDouble(...) call.
Use ResultSet.getObject(...), and test for null using ==.
The getObject method will deliver a (non-NULL) value as a Double (assuming that the column type is double). It is documented to return null for a NULL. (For more information, this page documents the default mappings of SQL types to Java types, and therefore what actual type you should expect getObject to deliver.)
I would recommend using a Double not a double as your type then you check against null.
A double primitive in Java can never be null. It will be initialized to 0.0 if no value has been given for it (except when declaring a local double variable and not assigning a value, but this will produce a compile-time error).
More info on default primitive values here.
To say that something "is null" means that it is a reference to the null value. Primitives (int, double, float, etc) are by definition not reference types, so they cannot have null values. You will need to find out what your database wrapper will do in this case.
How are you getting the value of "results"? Are you getting it via ResultSet.getDouble()? In that case, you can check ResultSet.wasNull().
I believe Double.NaN might be able to cover this. That is the only 'null' value double contains.