Does NULL have a type?

Question

Various sources (eg Wikipedia, PSOUG) state that Oracle's null does not have a type. Is this true?

What about other RDBMSs?

Answer 1

Oracle:

The null literal does not have a type, but

1. null can be cast to any type, and this may be necessary when

   • calling overloaded procedures or functions

   • controlling the return type of the decode function, eg:

     select decode('A','B',to_char(null),'A','1') from dual;
     DECODE('A','B',TO_CHAR(NULL),'A','1')
     -------------------------------------
     1
     
     select decode('A','B',to_number(null),'A','1') from dual;
     DECODE('A','B',TO_NUMBER(NULL),'A','1')
     --------------------------------------- 
                                           1
     

   • controlling column types of set operators like union when the first query block includes a null

2. null values stored in the database always have a type:

   create table t(n integer, s varchar(10));
   insert into t values(null, null);
   
   select decode('A','B',n,'A','1') from t; 
   DECODE('A','B',N,'A','1')
   -------------------------
                           1
   
   select decode('A','B',s,'A','1') from t;
   DECODE('A','B',S,'A','1')
   -------------------------
   1
   

Answer 2

SQL Server, int

SELECT NULL AS foo INTO dbo.bar
SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'bar'
DROP TABLE dbo.bar

MySQL, binary(0)

CREATE TABLE mydb.foo (select NULL AS bar);
EXPLAIN mydb.foo;
DROP TABLE mydb.foo;

Answer 3

Oracle it is in some sense some string type.

Thats what ADO Reader tells me. here is a Powershell Script:

[System.Reflection.Assembly]::LoadWithPartialName("System.Data.OracleClient") 
$ConnectionString = "Data Source=myTNS;User ID=myUSER;Password=myPassword" 
$conn=new-object System.Data.OracleClient.OracleConnection 
$conn.ConnectionString=$ConnectionString 
$conn.Open() 
$sql = "Select NULL xx from DUAL"
$cmd=new-object System.Data.OracleClient.OracleCommand($sql,$conn)

$r = $cmd.ExecuteReader()

$r.GetSchemaTable() | % { $_
}        

That gives

ColumnName               : XX
ColumnOrdinal            : 0
ColumnSize               : 0
NumericPrecision         : 0
NumericScale             : 0
DataType                 : System.String
ProviderType             : 22
IsLong                   : False
AllowDBNull              : True
IsAliased                : 
IsExpression             : 
IsKey                    : 
IsUnique                 : 
BaseSchemaName           : 
BaseTableName            : 
BaseColumnName           : 
ProviderSpecificDataType : System.Data.OracleClient.OracleString

Note the line

ProviderSpecificDataType : System.Data.OracleClient.OracleString

Answer 4

postgres:

create table foo as select null as bar;
WARNING:  column "bar" has type "unknown"
DETAIL:  Proceeding with relation creation anyway.

postgres=> \d foo

 Column |  Type   | Modifiers
--------+---------+-----------
 bar    | unknown |