mysql - NULL value in multi-column primary key

Question

I've got a table with several columns making up the primary key. The nature of the data stored allows some of these fields to have NULL values. I have designed my table as such:

CREATE TABLE `test` (
    `Field1` SMALLINT(5) UNSIGNED NOT NULL,
    `Field2` DECIMAL(5,2) UNSIGNED NULL DEFAULT NULL,
    PRIMARY KEY (`Field1`, `Field2`)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB;

However, when I run describe test it shows like this:

|| *Field* || *Type*                || *Null* || *Key* || *Default* || *Extra* 
|| Field1  || smallint(5) unsigned  || NO     || PRI   ||           ||         
|| Field2  || decimal(5,2) unsigned || NO     || PRI   || 0.00      ||         

And I keep getting an error when inserting a NULL value.

Column 'Field2' cannot be null

Is this because a field that is part of a primary key cannot be null? What are my alternatives besides using, say, '0' for NULL?

Answer 1

From the MySQL documentation :

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they
are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). A table can have only one PRIMARY KEY. The name of a PRIMARY KEY is always PRIMARY, which thus cannot be used as the name for any other kind of index.

http://dev.mysql.com/doc/refman/5.1/en/create-table.html

If Field2 can be NULL, I question why you need it as part of the Primary Key since you then need Field1 to be distinct across all rows. So Field1 by itself should be sufficient as the Primary Key. You could create a different type of index on Field2.