diff --git a/datamodel/app/symbology_functions.sql b/datamodel/app/symbology_functions.sql
index ab381e853..3fcbb9361 100644
--- a/datamodel/app/symbology_functions.sql
+++ b/datamodel/app/symbology_functions.sql
@@ -289,7 +289,7 @@ LANGUAGE plpgsql VOLATILE;
 --------------------------------------------------------
 -- UPDATE wastewater structure fk_main_cover
 -- Argument:
---  * obj_id of wastewater structure
+--  * obj_id of wastewater structure. No change if fk_main_cover is not null
 --  * all True to update all
 --  * omit both arguments to update all where fk_main_cover is null
 --------------------------------------------------------
@@ -308,7 +308,7 @@ BEGIN
       LEFT JOIN tww_od.structure_part sp ON sp.fk_wastewater_structure = ws.obj_id
       LEFT JOIN tww_od.cover co ON sp.obj_id = co.obj_id
       LEFT JOIN tww_od.channel ch ON ch.obj_id = ws.obj_id
-      WHERE ch.obj_id IS NULL AND (_all OR ws.obj_id = _obj_id OR ( NOT _all AND _obj_id is NULL AND ws.fk_main_cover IS NULL))
+      WHERE ch.obj_id IS NULL AND (_all OR ((ws.obj_id = _obj_id OR (NOT _all AND _obj_id is NULL)) AND ws.fk_main_cover IS NULL))
   ) ws_covers
   WHERE ws.obj_id = ws_covers.obj_id;
 END
@@ -320,7 +320,7 @@ VOLATILE;
 --------------------------------------------------------
 -- UPDATE wastewater structure fk_main_wastewater_node
 -- Argument:
---  * obj_id of wastewater structure
+--  * obj_id of wastewater structure. No change if fk_main_wastewater_node is not null
 --  * all True to update all
 --  * omit both arguments to update all where fk_main_wastewater_node is null
 --------------------------------------------------------
@@ -340,7 +340,7 @@ BEGIN
       LEFT JOIN tww_od.wastewater_networkelement ne ON ne.fk_wastewater_structure = ws.obj_id
       LEFT JOIN tww_od.wastewater_node wn ON ne.obj_id = wn.obj_id
       LEFT JOIN tww_od.channel ch ON ch.obj_id = ws.obj_id
-      WHERE ch.obj_id IS NULL AND (_all OR ws.obj_id = _obj_id OR ( NOT _all AND _obj_id is NULL AND ws.fk_main_wastewater_node IS NULL))
+      WHERE ch.obj_id IS NULL AND (_all OR ((ws.obj_id = _obj_id OR (NOT _all AND _obj_id is NULL)) AND ws.fk_main_wastewater_node IS NULL))
   ) ws_nodes
   WHERE ws.obj_id = ws_nodes.obj_id;
 END