diff --git a/CHANGELOG.md b/CHANGELOG.md
index a8571c3e721f27d97cea76b79a6e7eeb9ceeb1bc..c9bfbe77011496e878d6aacf70ffba6e93125b5d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed ###
 
+* Bug in applyPOV which led to caosdb-server#154: Queries with "!=" returned
+  the same results as "=" (for references).
+
 ### Security ###
 
 ## [4.1.0] - 2021-06-11 ##
diff --git a/procedures/query/applyPOV.sql b/procedures/query/applyPOV.sql
index 732854be91b5dcda817da3faa00c9ba727a31e3e..a6608fa1e9c294ce8abd7ea54d6b14699f2ae34e 100644
--- a/procedures/query/applyPOV.sql
+++ b/procedures/query/applyPOV.sql
@@ -82,6 +82,7 @@ POV_LABEL: BEGIN
     DECLARE distinctUnits INT DEFAULT 0;
     DECLARE usedStdUnit BIGINT DEFAULT NULL;
     DECLARE keepTabl VARCHAR(255) DEFAULT NULL;
+    DECLARE existence_op VARCHAR(255) DEFAULT "EXISTS";
 
     IF o = '->' THEN
         #-- special case: pure reference property
@@ -210,17 +211,32 @@ POV_LABEL: BEGIN
             ));
             SET sNameData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `name_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_name_data` ', CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `name_data` AS subdata WHERE subdata.value ', o, ' ? UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_name_data` AS subdata WHERE subdata.value ', o, '?'));
             SET sEnumData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `enum_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_enum_data` ', CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `enum_data` AS subdata WHERE subdata.value ', o, ' ? UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_enum_data` AS subdata WHERE subdata.value ', o, '?'));
-            SET sRefData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `reference_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_reference_data`', IF(refIdsTable IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `reference_data` AS subdata WHERE EXISTS (SELECT 1 FROM `', refIdsTable, '` AS refIdsTable WHERE subdata.value=refIdsTable.id LIMIT 1) UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_reference_data` AS subdata WHERE EXISTS (SELECT 1 FROM `', refIdsTable, '` AS refIdsTable WHERE subdata.value=refIdsTable.id LIMIT 1)')));
+            IF o = "!=" AND refIdsTable IS NOT NULL THEN
+                SET existence_op = "NOT EXISTS";
+            END IF;
+            SET sRefData = IF(vText IS NULL,
+                ' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `reference_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_reference_data`',
+                IF(refIdsTable IS NULL,
+                    NULL,
+                    CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `reference_data` AS subdata WHERE ', existence_op, ' (SELECT 1 FROM `', refIdsTable, '` AS refIdsTable WHERE subdata.value=refIdsTable.id LIMIT 1) AND subdata.status != "REPLACEMENT" UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_reference_data` AS subdata WHERE ', existence_op, ' (SELECT 1 FROM `', refIdsTable, '` AS refIdsTable WHERE subdata.value=refIdsTable.id LIMIT 1) AND subdata.status != "REPLACEMENT"')));
             SET sDoubleData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT subdata.domain_id, subdata.entity_id, _get_head_iversion(subdata.entity_id) AS _iversion, subdata.property_id FROM `double_data` AS subdata UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_double_data` ', IF(vDouble IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id), property_id FROM `double_data` AS subdata WHERE ', getDoubleWhereClause(vDouble,unit_sig,vDoubleStdUnit,stdUnit_sig,o), ' UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_double_data` AS subdata WHERE ', getDoubleWhereClause(vDouble, unit_sig, vDoubleStdUnit, stdUnit_sig, o))));
             SET sIntData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT subdata.domain_id, subdata.entity_id, _get_head_iversion(subdata.entity_id) AS _iversion, subdata.property_id FROM `integer_data` AS subdata UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_integer_data`', IF(vInt IS NULL AND vDoubleStdUnit IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `integer_data` AS subdata WHERE ', getDoubleWhereClause(vInt, unit_sig, vDoubleStdUnit, stdUnit_sig, o), ' UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_integer_data` AS subdata WHERE ', getDoubleWhereClause(vInt, unit_sig, vDoubleStdUnit, stdUnit_sig, o))));
             SET sDatetimeData = IF(vText IS NULL,' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `datetime_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_datetime_data`', IF(vDateTime IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `datetime_data` AS subdata WHERE ',getDateTimeWhereClause(vDateTime,o), ' UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_datetime_data` AS subdata WHERE ',getDateTimeWhereClause(vDateTime,o))));
             SET sDateData = IF(vText IS NULL,' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `date_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_date_data`', IF(vDateTimeDotNotation IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `date_data` AS subdata WHERE ', getDateWhereClause(vDateTimeDotNotation,o), ' UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_date_data` AS subdata WHERE ', getDateWhereClause(vDateTimeDotNotation,o))));
             SET sNullData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, _get_head_iversion(entity_id) AS _iversion, property_id FROM `null_data` UNION ALL SELECT DISTINCT domain_id, entity_id, _iversion, property_id FROM `archive_null_data`', NULL);
+        -- unversioned query --------------------------------------------
         ELSE
             SET sTextData = IF(vText IS NULL, 'SELECT DISTINCT domain_id, entity_id, property_id FROM `text_data`', CONCAT('SELECT DISTINCT domain_id, entity_id, property_id FROM `text_data` AS subdata WHERE subdata.value ',o,' ?'));
             SET sNameData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `name_data`', CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `name_data` AS subdata WHERE subdata.value ', o, ' ?'));
             SET sEnumData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `enum_data`', CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `enum_data` AS subdata WHERE subdata.value ', o, ' ?'));
-            SET sRefData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `reference_data`', IF(refIdsTable IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `reference_data` AS subdata WHERE EXISTS (SELECT 1 FROM `', refIdsTable, '` AS refIdsTable WHERE subdata.value=refIdsTable.id LIMIT 1)')));
+            IF o = "!=" AND refIdsTable IS NOT NULL THEN
+                SET existence_op = "NOT EXISTS";
+            END IF;
+            SET sRefData = IF(vText IS NULL,
+                ' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `reference_data`',
+                IF(refIdsTable IS NULL,
+                    NULL,
+                    CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `reference_data` AS subdata WHERE ',existence_op ,' (SELECT 1 FROM `', refIdsTable, '` AS refIdsTable WHERE subdata.value=refIdsTable.id LIMIT 1) AND subdata.status != "REPLACEMENT"')));
             SET sDoubleData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT subdata.domain_id, subdata.entity_id, subdata.property_id FROM `double_data` AS subdata', IF(vDouble IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `double_data` AS subdata WHERE ', getDoubleWhereClause(vDouble,unit_sig,vDoubleStdUnit,stdUnit_sig,o))));
             SET sIntData = IF(vText IS NULL, ' UNION ALL SELECT DISTINCT subdata.domain_id, subdata.entity_id, subdata.property_id FROM `integer_data` AS subdata', IF(vInt IS NULL AND vDoubleStdUnit IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `integer_data` AS subdata WHERE ', getDoubleWhereClause(vInt, unit_sig, vDoubleStdUnit, stdUnit_sig, o))));
             SET sDatetimeData = IF(vText IS NULL,' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `datetime_data`', IF(vDateTime IS NULL, NULL, CONCAT(' UNION ALL SELECT DISTINCT domain_id, entity_id, property_id FROM `datetime_data` AS subdata WHERE ',getDateTimeWhereClause(vDateTime,o))));