Fix NPE in Parser.parseMergeUsing()

h2/src/main/org/h2/command/Parser.java

@@ -1135,19 +1135,23 @@ public class Parser {
                 command.setQuery(parseSelect());
                 read(")");
             }
-            command.setQueryAlias(readFromAlias(null, Collections.singletonList("ON")));
+            String queryAlias = readFromAlias(null, Collections.singletonList("ON"));
+            if (queryAlias == null) {
+                queryAlias = Constants.PREFIX_QUERY_ALIAS + parseIndex;
+            }
+            command.setQueryAlias(queryAlias);
 
             String[] querySQLOutput = {null};
             List<Column> columnTemplateList = TableView.createQueryColumnTemplateList(null, command.getQuery(),
                     querySQLOutput);
             TableView temporarySourceTableView = createCTEView(
-                    command.getQueryAlias(), querySQLOutput[0],
+                    queryAlias, querySQLOutput[0],
                     columnTemplateList, false/* no recursion */,
                     false/* do not add to session */,
                     false /* isPersistent */,
                     session);
             TableFilter sourceTableFilter = new TableFilter(session,
-                    temporarySourceTableView, command.getQueryAlias(),
+                    temporarySourceTableView, queryAlias,
                     rightsChecked, (Select) command.getQuery(), 0, null);
             command.setSourceTableFilter(sourceTableFilter);
         } else {

h2/src/main/org/h2/engine/Constants.java

@@ -386,6 +386,11 @@ public class Constants {
      */
     public static final String PREFIX_PRIMARY_KEY = "PRIMARY_KEY_";
 
+    /**
+     * The name prefix used for query aliases that are not explicitly named.
+     */
+    public static final String PREFIX_QUERY_ALIAS = "QUERY_ALIAS_";
+
     /**
      * Every user belongs to this role.
      */

h2/src/test/org/h2/test/scripts/dml/mergeUsing.sql

@@ -147,6 +147,18 @@ SELECT * FROM TEST ORDER BY C1, C2;
 > 11 22 32
 > rows (ordered): 2
 
+MERGE INTO TEST USING (SELECT 1 FROM DUAL) ON (C1 = 11 AND C2 = 21)
+    WHEN NOT MATCHED THEN INSERT (C1, C2, C3) VALUES (11, 21, 33)
+    WHEN MATCHED THEN UPDATE SET C3 = 34;
+> update count: 1
+
+SELECT * FROM TEST ORDER BY C1, C2;
+> C1 C2 C3
+> -- -- --
+> 11 21 34
+> 11 22 32
+> rows (ordered): 2
+
 DROP TABLE TEST;
 > ok