DB2: Won't Allow “NULL” column?

问题

Part of a complex query that our app is running contains the lines: ...(inner query)

SELECT
...
NULL as column_A,
NULL as column_B,
...
FROM
...

This syntax of creating columns with null values is not allowed in DB2 altough it is totally OK in MSSQL and Oracle DBs. Technically I can change it to:

'' as column_A,
'' as column_B, 

But this doesn't have exactly the same meaning and can damage our calculation results. How can I create columns with null values in DB2 using other syntax??

回答1:

DB2 is strongly typed, so you need to tell DB2 what kind of column your NULL is:

select 
   ...
   cast(NULL as int) as column_A,
   cast(NULL as varchar(128)) as column_B,
   ...
FROM
   ...

回答2:

For Db2 LUW, since version 9.7, you can (if you like) use the NULL value without explicitly casting it to a particular data type.

NULL on it's own will be implicitly cast to VARCHAR(1). This knowledge center page will show what happens in other cases: Determining data types of untyped expressions

Examples

db2 "describe values ( NULL, + NULL, NULL || NULL  )"

 Column Information

 Number of columns: 3

 SQL type              Type length  Column name                     Name length
 --------------------  -----------  ------------------------------  -----------
 449   VARCHAR                   1  1                                         1
 997   DECFLOAT                 16  2                                         1
 449   VARCHAR                 508  3                                         1

and

db2 "describe values NULL, 1"

 Column Information

 Number of columns: 1

 SQL type              Type length  Column name                     Name length
 --------------------  -----------  ------------------------------  -----------
 497   INTEGER                   4  1                                         1

来源：https://stackoverflow.com/questions/2509233/db2-wont-allow-null-column