STRING_AGG except specific string from another column

Question

I wanna know how to exclude specific string in STRING_AGG? Here's my original query I want to exclude from the concatenation for a specific value on column name PRIMARY. Sorry I don't know how to use correct format, I will attach image instead for clear picture of my scenario.

SELECT t1.ID, t1.NAME, STRING_AGG(Trim(t2.COUNTRY), ';') WITHIN GROUP(ORDER BY COUNTRY) COUNTRY, PRIMARY FROM t1 LEFT JOIN t2 on t1.ID=t2.ID GROUP BY t1.ID,t1.NAME,t1.PRIMARY

t1

ID NAME PRIMARY 1 WHITE US 2 RED CA 3 BLUE US 4 GREEN GB

t2 ID COUNTRY 1 US 1 CA 2 GB 2 DE 3 DE 3 CA 4 CA 4 DE 4 US

RESULT: ID NAME PRIMARY COUNTRY 1 WHITE US CA (EXCEPT US BECAUSE IT'S ALREADY IN PRIMARY) 2 RED CA GB;DE
3 BLUE US DE;CA
4 GREEN DE CA;DE;US (EXCEPT DE BECAUSE IT'S ALREADY IN PRIMARY)

Answer 1

Just use a case expression:

STRING_AGG(CASE WHEN t2.Country <> Primary THEN Trim(t2.COUNTRY) END, ';') WITHIN GROUP (ORDER BY COUNTRY)

STRING_AGG() ignores NULL values.