diff --git a/src/main/java/io/redlink/more/data/controller/ExternalDataApiV1Controller.java b/src/main/java/io/redlink/more/data/controller/ExternalDataApiV1Controller.java
index 33d510f..f603cdb 100644
--- a/src/main/java/io/redlink/more/data/controller/ExternalDataApiV1Controller.java
+++ b/src/main/java/io/redlink/more/data/controller/ExternalDataApiV1Controller.java
@@ -13,7 +13,7 @@
 import io.redlink.more.data.api.app.v1.model.ParticipantDTO;
 import io.redlink.more.data.api.app.v1.webservices.ExternalDataApi;
 import io.redlink.more.data.controller.transformer.DataTransformer;
-import io.redlink.more.data.controller.transformer.StudyTransformer;
+import io.redlink.more.data.controller.transformer.ParticipantTransformer;
 import io.redlink.more.data.exception.BadRequestException;
 import io.redlink.more.data.model.ApiRoutingInfo;
 import io.redlink.more.data.model.RoutingInfo;
@@ -51,7 +51,7 @@ public ResponseEntity<List<ParticipantDTO>> listParticipants(String moreApiToken
             return ResponseEntity.ok(
                     externalService.listParticipants(apiRoutingInfo.studyId(), apiRoutingInfo.studyGroupId())
                             .stream()
-                            .map(StudyTransformer::toDTO)
+                            .map(ParticipantTransformer::toDTO)
                             .toList()
             );
         } catch(IndexOutOfBoundsException | NumberFormatException e) {
diff --git a/src/main/java/io/redlink/more/data/controller/transformer/ParticipantTransformer.java b/src/main/java/io/redlink/more/data/controller/transformer/ParticipantTransformer.java
new file mode 100644
index 0000000..c59ade1
--- /dev/null
+++ b/src/main/java/io/redlink/more/data/controller/transformer/ParticipantTransformer.java
@@ -0,0 +1,23 @@
+package io.redlink.more.data.controller.transformer;
+
+import io.redlink.more.data.api.app.v1.model.ParticipantDTO;
+import io.redlink.more.data.api.app.v1.model.ParticipantStatusDTO;
+import io.redlink.more.data.model.Participant;
+
+public final class ParticipantTransformer {
+
+    private ParticipantTransformer() {}
+
+    public static ParticipantDTO toDTO(Participant participant) {
+        if(participant == null) {
+            return null;
+        }
+        return new ParticipantDTO(
+                String.valueOf(participant.id()),
+                participant.alias(),
+                ParticipantStatusDTO.fromValue(participant.status()),
+                participant.studyGroupId(),
+                BaseTransformers.toOffsetDateTime(participant.start())
+        );
+    }
+}
diff --git a/src/main/java/io/redlink/more/data/controller/transformer/StudyTransformer.java b/src/main/java/io/redlink/more/data/controller/transformer/StudyTransformer.java
index 801b3e4..d46a2df 100644
--- a/src/main/java/io/redlink/more/data/controller/transformer/StudyTransformer.java
+++ b/src/main/java/io/redlink/more/data/controller/transformer/StudyTransformer.java
@@ -41,19 +41,6 @@ public static SimpleParticipantDTO toDTO(SimpleParticipant participant) {
                 .alias(participant.alias());
     }
 
-    public static ParticipantDTO toDTO(Participant participant) {
-        if(participant == null) {
-            return null;
-        }
-        return new ParticipantDTO(
-                participant.id(),
-                participant.alias(),
-                ParticipantStatusDTO.fromValue(participant.status()),
-                participant.studyGroupId(),
-                BaseTransformers.toOffsetDateTime(participant.start())
-        );
-    }
-
     public static ContactInfoDTO toDTO(Contact contact) {
         return new ContactInfoDTO()
                 .institute(contact.institute())
diff --git a/src/main/java/io/redlink/more/data/repository/StudyRepository.java b/src/main/java/io/redlink/more/data/repository/StudyRepository.java
index 09b52cf..76de2ca 100644
--- a/src/main/java/io/redlink/more/data/repository/StudyRepository.java
+++ b/src/main/java/io/redlink/more/data/repository/StudyRepository.java
@@ -84,11 +84,7 @@ public class StudyRepository {
 
     private static final String SQL_LIST_PARTICIPANTS_BY_STUDY =
             "SELECT participant_id, alias, status, study_group_id, start FROM participants " +
-            "WHERE study_id = ?";
-
-    private static final String SQL_LIST_PARTICIPANTS_BY_STUDY_AND_GROUP =
-            "SELECT participant_id, alias, status, study_group_id, start FROM participants " +
-            "WHERE study_id = ? AND study_group_id = ?";
+            "WHERE study_id = :study_id AND (study_group_id = :study_group_id OR :study_group_id::INT IS NULL)";
 
     private static final String GET_OBSERVATION_PROPERTIES_FOR_PARTICIPANT =
             "SELECT properties FROM participant_observation_properties " +
@@ -202,9 +198,19 @@ public Optional<SimpleParticipant> findParticipant(RoutingInfo routingInfo) {
 
     public List<Participant> listParticipants(long studyId, int groupId) {
         if(groupId < 0) {
-            return jdbcTemplate.query(SQL_LIST_PARTICIPANTS_BY_STUDY, getParticipantRowMapper(), studyId);
+            return namedTemplate.query(
+                    SQL_LIST_PARTICIPANTS_BY_STUDY,
+                    new MapSqlParameterSource()
+                            .addValue("study_id", studyId)
+                            .addValue("study_group_id", null),
+                    getParticipantRowMapper());
         } else {
-            return jdbcTemplate.query(SQL_LIST_PARTICIPANTS_BY_STUDY_AND_GROUP, getParticipantRowMapper(), studyId, groupId);
+            return namedTemplate.query(
+                    SQL_LIST_PARTICIPANTS_BY_STUDY,
+                    new MapSqlParameterSource()
+                            .addValue("study_id", studyId)
+                            .addValue("study_group_id", groupId),
+                    getParticipantRowMapper());
         }
     }
 
diff --git a/src/main/resources/openapi/ExternalAPI.yaml b/src/main/resources/openapi/ExternalAPI.yaml
index 2fb6ea8..b125496 100644
--- a/src/main/resources/openapi/ExternalAPI.yaml
+++ b/src/main/resources/openapi/ExternalAPI.yaml
@@ -113,8 +113,7 @@ components:
       description: A participant for a study
       properties:
         participantId:
-          type: integer
-          format: int32
+          type: string
         alias:
           type: string
         status: