should be it

external/duckdb/test/sql/upsert/upsert_unique_null.test

@@ -0,0 +1,37 @@
+# name: test/sql/upsert/upsert_unique_null.test
+# group: [upsert]
+
+statement ok
+pragma enable_verification;
+
+statement ok
+create or replace table tbl (
+	a integer unique,
+	b integer
+);
+
+statement ok
+insert into tbl VALUES (3,2), (1,3);
+
+statement ok
+insert into tbl(b) VALUES (5) ON CONFLICT (a) DO UPDATE SET b = 8;
+
+# UNIQUE can be NULL, PRIMARY KEY can't be
+query II
+select * from tbl;
+----
+3	2
+1	3
+NULL	5
+
+statement ok
+insert into tbl(b) VALUES (5) ON CONFLICT (a) DO UPDATE SET b = 8;
+
+# And NULL does not take part in the UNIQUE Index, so it does not activate the ON CONFLICT clause
+query II
+select * from tbl;
+----
+3	2
+1	3
+NULL	5
+NULL	5