can a foreign key allow null

Hi All,

I have two tables.

MYDB=# \d myschema.tbl_part
                Table "myschema.tbl_part"
        Column        |          Type          | Modifiers
----------------------+------------------------+-----------
 id                   | character varying(20)  | not null
 description          | character varying(30)  | not null
 class                | smallint               | not null
 inactive             | boolean                | not null
 sales_description    | character varying(160) |
 purchase_description | character varying(160) |
 last_unit_cost       | real                   | not null
 costing_method       | smallint               | not null
 sales_gl_account     | character varying(15)  |
 inventory_gl_account | character varying(15)  |
 cogs_gl_account      | character varying(15)  |
 type                 | character varying(8)   |
 unit_of_measure      | character varying(6)   |
 weight               | real                   |
 reorder_point        | real                   |
 reorder_quantity     | real                   |
Indexes: tbl_part_pkey primary key btree (id)

MYDB=# \d myschema.tbl_gl_account
        Table "myschema.tbl_gl_account"
   Column    |         Type          | Modifiers
-------------+-----------------------+-----------
 number      | character varying(15) | not null
 description | character varying(30) |
 type        | smallint              | not null
 inactive    | boolean               | not null
Indexes: tbl_gl_account_pkey primary key btree (number)

MYDB=#

Is it possible to create a foreign key such that only null and values from
myschema.tbl_gl_account.number are allowed in myschema.tbl_part.sales_gl_account?

Kind Regards,
Keith

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