fix: Cypher MERGE with label-only pattern now finds existing nodes, closes #3253

engine/src/main/java/com/arcadedb/query/opencypher/executor/steps/MergeStep.java

@@ -306,24 +306,32 @@ private boolean mergePath(final PathPattern pathPattern, final ResultInternal re
    * @return matching vertex or null
    */
   private Vertex findNode(final NodePattern nodePattern, final Result result) {
-    if (!nodePattern.hasLabels() || !nodePattern.hasProperties()) {
-      // Can't match without label and properties
+    if (!nodePattern.hasLabels()) {
+      // Can't match without a label
       return null;
     }
 
     final String label = nodePattern.getFirstLabel();
+
+    // Check if the type exists in the schema before iterating
+    if (!context.getDatabase().getSchema().existsType(label)) {
+      return null;
+    }
+
     @SuppressWarnings("unchecked")
     final Iterator<Identifiable> iterator = (Iterator<Identifiable>) (Object) context.getDatabase().iterateType(label, true);
 
-    // Evaluate property expressions against current result context
-    final Map<String, Object> evaluatedProperties = evaluateProperties(nodePattern.getProperties(), result);
+    // Evaluate property expressions against current result context (may be empty)
+    final Map<String, Object> evaluatedProperties = nodePattern.hasProperties()
+        ? evaluateProperties(nodePattern.getProperties(), result)
+        : null;
 
-    // Find first vertex matching all properties
+    // Find first vertex matching all properties (or any vertex if no properties specified)
     while (iterator.hasNext()) {
       final Identifiable identifiable = iterator.next();
       if (identifiable instanceof Vertex) {
         final Vertex vertex = (Vertex) identifiable;
-        if (matchesProperties(vertex, evaluatedProperties)) {
+        if (evaluatedProperties == null || matchesProperties(vertex, evaluatedProperties)) {
           return vertex;
         }
       }

engine/src/test/java/com/arcadedb/query/opencypher/OpenCypherMergeTest.java

@@ -161,6 +161,66 @@ void testMergeRelationship() {
     assertThat(count).isEqualTo(1);
   }
 
+  /**
+   * Test that MERGE with label only (no properties) finds existing node instead of creating duplicates.
+   * This is the pattern: MERGE (n:PIPELINE_CONFIG) ON CREATE SET n.pipelines = ["miaou"] ON MATCH SET n.pipelines = ["miaou"]
+   */
+  @Test
+  void testMergeLabelOnlyFindsExistingNode() {
+    database.getSchema().createVertexType("PIPELINE_CONFIG");
+
+    // First MERGE should create the node
+    database.transaction(() -> {
+      final ResultSet result = database.command("opencypher",
+          "MERGE (n:PIPELINE_CONFIG) ON CREATE SET n.pipelines = ['miaou'] ON MATCH SET n.pipelines = ['miaou'] RETURN n.pipelines as pipelines");
+      assertThat(result.hasNext()).isTrue();
+      result.next();
+    });
+
+    // Second MERGE should find the existing node, not create a duplicate
+    database.transaction(() -> {
+      final ResultSet result = database.command("opencypher",
+          "MERGE (n:PIPELINE_CONFIG) ON CREATE SET n.pipelines = ['miaou'] ON MATCH SET n.pipelines = ['miaou'] RETURN n.pipelines as pipelines");
+      assertThat(result.hasNext()).isTrue();
+      result.next();
+    });
+
+    // Verify only one node exists
+    final ResultSet verify = database.query("opencypher",
+        "MATCH (n:PIPELINE_CONFIG) RETURN n");
+    int count = 0;
+    while (verify.hasNext()) {
+      verify.next();
+      count++;
+    }
+    assertThat(count).isEqualTo(1);
+  }
+
+  /**
+   * Test that MERGE with label only (no properties) correctly triggers ON CREATE SET on first call
+   * and ON MATCH SET on subsequent calls.
+   */
+  @Test
+  void testMergeLabelOnlyWithOnCreateAndOnMatchSet() {
+    database.getSchema().createVertexType("SINGLETON");
+
+    // First MERGE should create and apply ON CREATE SET
+    ResultSet result = database.command("opencypher",
+        "MERGE (n:SINGLETON) ON CREATE SET n.status = 'created', n.count = 1 ON MATCH SET n.status = 'matched', n.count = 2 RETURN n");
+    assertThat(result.hasNext()).isTrue();
+    Vertex v = (Vertex) result.next().getProperty("n");
+    assertThat(v.get("status")).isEqualTo("created");
+    assertThat(((Number) v.get("count")).intValue()).isEqualTo(1);
+
+    // Second MERGE should match and apply ON MATCH SET
+    result = database.command("opencypher",
+        "MERGE (n:SINGLETON) ON CREATE SET n.status = 'created', n.count = 1 ON MATCH SET n.status = 'matched', n.count = 2 RETURN n");
+    assertThat(result.hasNext()).isTrue();
+    v = (Vertex) result.next().getProperty("n");
+    assertThat(v.get("status")).isEqualTo("matched");
+    assertThat(((Number) v.get("count")).intValue()).isEqualTo(2);
+  }
+
   /**
    * Test for issue #3217: Backticks in relationship types should be treated as escape characters,
    * not included in the relationship type name.