Enforcing a unique relationship over multiple columns where one column is nullable - db2

Question

Given the table
ID PERSON_ID PLAN EMPLOYER_ID TERMINATION_DATE
1 123 ABC 321 2020-01-01
2 123 DEF 321 (null)
3 123 ABC 321 (null)
4 123 ABC 321 (null)
I want to exclude the 4th entry. (The 3rd entry shows the person was re-hired and therefore is a new relationship. I'm only showing relevant fields)
My first attempt was to simply create a unique index over PERSON_ID / PLAN / EMPLOYER_ID / TERMINATION_DATE, thinking that DB2 for IBMi considered nulls equal in a unique index. I was evidently wrong...
Is there a way to enforce uniqueness over these columns, or,
is there a better way to approach the value of termination date? (null is not technically correct; I'm thinking of it as more true/false, but the business logic needs a date)
Edit
According to the docs for 7.3:
UNIQUE
Prevents the table from containing two or more rows with the same value of the index key. When UNIQUE is used, all null values for a column are considered equal. For example, if the key is a single column that can contain null values, that column can contain only one null value. The constraint is enforced when rows of the table are updated or new rows are inserted.
The constraint is also checked during the execution of the CREATE INDEX statement. If the table already contains rows with duplicate key values, the index is not created.
UNIQUE WHERE NOT NULL
Prevents the table from containing two or more rows with the same value of the index key, where all null values for a column are not considered equal. Multiple null values in a column are allowed. Otherwise, this is identical to UNIQUE.
So, the behavior I'm seeing looks more like UNIQUE WHERE NOT NULL. When I generate SQL for this table, I see
ADD CONSTRAINT TERMEMPPLANSSN
UNIQUE( TERMINATION_DATE , EMPLOYERID , PLAN_CODE , SSN ) ;
(note this is showing the real field names, not the ones I used in my example)
Edit 2
Bottom line, Constraint !== Index. When I went back and created an actual index, I got the desired behavior.

Answer 1

CREATE TABLE PERSON
(
ID INT NOT NULL
, PERSON_ID INT NOT NULL
, PLAN CHAR(3) NOT NULL
, EMPLOYER_ID INT
, TERMINATION_DATE DATE
);
INSERT INTO PERSON (ID, PERSON_ID, PLAN, EMPLOYER_ID, TERMINATION_DATE)
VALUES
(1, 123, 'ABC', 321, DATE('2020-01-01'))
, (2, 123, 'DEF', 321, CAST(NULL AS DATE))
, (3, 123, 'ABC', 321, CAST(NULL AS DATE))
WITH NC;
--- To not allow: ---
INSERT INTO PERSON (ID, PERSON_ID, PLAN, EMPLOYER_ID, TERMINATION_DATE) VALUES
(4, 123, 'ABC', 321, CAST(NULL AS DATE))
or
(4, 123, 'ABC', 321, DATE('2020-01-01'))
You may:
CREATE UNIQUE INDEX PERSON_U1 ON PERSON
(PERSON_ID, PLAN, EMPLOYER_ID, TERMINATION_DATE);
--- To not allow: ---
INSERT INTO PERSON (ID, PERSON_ID, PLAN, EMPLOYER_ID, TERMINATION_DATE) VALUES
(4, 123, 'ABC', 321, DATE('2020-01-01'))
but allow multiple:
(X, 123, 'ABC', 321, CAST(NULL AS DATE))
(Y, 123, 'ABC', 321, CAST(NULL AS DATE))
...
You may:
CREATE UNIQUE WHERE NOT NULL INDEX PERSON_U2 ON PERSON
(PERSON_ID, PLAN, EMPLOYER_ID, TERMINATION_DATE);