NULL value in multi-column primary key

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?

From the MySQL documentation :

> PRIMARY KEY > > 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.

https://dev.mysql.com/doc/refman/8.0/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.

>Primary keys are used to make the column both unique and not null

In order to insert NULL values make field2 as UNIQUE.

>Unique constraint makes the field removes duplicates but allows null values

Primary key states that column mustn't have NULL values. So columns used for defining composite primary key isn't going to be NULL.

Also Oracle server compares the combination of all columns used in a composite primary key definition. If your all columns existing data (say x,y) matched with newly adding row, it will raise error of Unique Constraint Violated.

Moreover,look at this thread: https://stackoverflow.com/questions/386040/whats-wrong-with-nullable-columns-in-composite-primary-keys.

This link provides valuable information regarding possibility of NULLABLE columns in composite key!

You can use unique key like this:

mysql> CREATE TABLE `test` (
    ->     `Field1` SMALLINT(5) UNSIGNED NOT NULL,
    ->     `Field2` DECIMAL(5,2) UNSIGNED NULL DEFAULT NULL,
    ->     UNIQUE KEY (`Field1`, `Field2`)
    -> )
    -> COLLATE='latin1_swedish_ci'
    -> ENGINE=InnoDB;
Query OK, 0 rows affected (0.03 sec)

mysql> 
mysql> desc test
    -> ;
+--------+-----------------------+------+-----+---------+-------+
| Field  | Type                  | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+-------+
| Field1 | smallint(5) unsigned  | NO   | MUL | NULL    |       |
| Field2 | decimal(5,2) unsigned | YES  |     | NULL    |       |
+--------+-----------------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

you can use unique keys, please take a look to this link, they work with null values

http://www.xaprb.com/blog/2009/09/12/the-difference-between-a-unique-index-and-primary-key-in-mysql/