diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1086d15b9..925415e05 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,19 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ## Unreleased
 
+### Added
+
+- SQL status code in error message
+- All transactional errors cause setting of `ProblemHint.RETRY_RECOMMENDED`
+
+### Fixed
+
+- Check constraint on table `subjects` obeys empty `identifiers`:
+  If there are no identifiers then type and label must be unique
+  among those entries without identifiers. Therefore entries are allowed
+  that have the same type and label but one is with and one w/o
+  identifiers.
+
 ## [9.4.1](https://github.com/dbmdz/metadata-service/releases/tag/9.4.1) - 2025-01-09
 
 ### Fixed
diff --git a/metasvc-server/backend-jdbi/src/main/resources/io/github/dbmdz/metadata/server/backend/impl/database/migration/R__func_subjects_is_unique_type_identifier.sql b/metasvc-server/backend-jdbi/src/main/resources/io/github/dbmdz/metadata/server/backend/impl/database/migration/R__func_subjects_is_unique_type_identifier.sql
new file mode 100644
index 000000000..3e53240f8
--- /dev/null
+++ b/metasvc-server/backend-jdbi/src/main/resources/io/github/dbmdz/metadata/server/backend/impl/database/migration/R__func_subjects_is_unique_type_identifier.sql
@@ -0,0 +1,36 @@
+CREATE OR REPLACE FUNCTION public.subjects_is_unique_type_identifier(puuid uuid, ptype character varying, ids dbidentifier[], plabel jsonb)
+RETURNS boolean
+STABLE LANGUAGE plpgsql
+AS $function$
+/**
+ * Ensures uniqueness of `type` and single identifier across table `subjects`.
+ *
+ * If there are not any identifiers then `type` and `label` must be unique.
+ * It is intended for check constraints and
+ * returns `false` on violation.
+ */
+declare
+	db_ident record; --dbidentifier actually but makes trouble in backups
+	tuple_exists boolean;
+begin
+	if ids is null or cardinality(ids) = 0 then
+		execute 'select exists(select 1 from public.subjects where $1 = type and $2 = label and coalesce(identifiers, array[]::dbidentifier[]) = array[]::dbidentifier[] and $3 <> uuid)'
+		  into tuple_exists
+		  using ptype, plabel, puuid;
+		if tuple_exists then
+			return false;
+		end if;
+	else
+		foreach db_ident in array ids loop
+			execute 'select exists(select 1 from public.subjects where $1 = type and $2 = any (identifiers) and $3 <> uuid)'
+			  into tuple_exists
+			  using ptype, db_ident, puuid;
+			if tuple_exists then
+				return false;
+			end if;
+		end loop;
+	end if;
+	return true;
+end;
+$function$;
+