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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