dialects fix

dialect/db/h2/src/main/java/org/eclipse/daanse/sql/dialect/db/h2/H2Dialect.java

@@ -188,12 +188,17 @@ public java.util.Optional<String> generateListAgg(CharSequence operand, boolean
         return java.util.Optional.of((buf).toString());
     }
 
-    /** The n-th value of the group's rows in their order: an ordered array, its n-th element (V-82). */
+    /**
+     * The n-th value of the group's rows in their order: an ordered array, its n-th element (V-82). H2 fails an
+     * element reference past the end ("Array element error", also ARRAY_GET), so the element is sliced out and a NULL
+     * appended: a group with fewer values gives NULL, as the aggregate promises.
+     */
     @Override
     public java.util.Optional<String> generateNthValueAgg(CharSequence operand, boolean ignoreNulls, Integer n,
             List<OrderedColumn> columns) {
-        return java.util.Optional.of(buildNthValueOfArray("ARRAY_AGG", operand, ignoreNulls, columns, "(",
-                ")[" + AggregationGenerator.nthIndex(n) + "]").toString());
+        int index = AggregationGenerator.nthIndex(n);
+        return java.util.Optional.of(buildNthValueOfArray("ARRAY_AGG", operand, ignoreNulls, columns, "(ARRAY_SLICE(",
+                ", " + index + ", " + index + ") || ARRAY[NULL])[1]").toString());
     }
 
     @Override

dialect/db/h2/src/test/java/org/eclipse/daanse/sql/dialect/db/h2/H2NthValuePastTheEndTest.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation.
+ *
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *   SmartCity Jena - initial
+ */
+package org.eclipse.daanse.sql.dialect.db.h2;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.eclipse.daanse.sql.dialect.api.DialectInitData;
+import org.eclipse.daanse.sql.model.sql.OrderedColumn;
+import org.eclipse.daanse.sql.model.sql.SortDirection;
+import org.junit.jupiter.api.Test;
+
+/**
+ * The n-th value aggregate runs on H2 and gives NULL for a group with fewer values: H2 fails an array element past
+ * the end, so the n-th value of four values asked for the fifth broke the query (TheNthValueIsPerCellTest on H2,
+ * database run 18.10.2026).
+ */
+class H2NthValuePastTheEndTest {
+
+    private static List<Object> nth(int n, String rows) throws SQLException {
+        try (Connection connection = DriverManager.getConnection("jdbc:h2:mem:nth" + n)) {
+            H2Dialect dialect = new H2Dialect(DialectInitData.fromConnection(connection));
+            String nth = dialect.generateNthValueAgg("\"x\"", false, n,
+                    List.of(new OrderedColumn("x", null, SortDirection.DESC))).orElseThrow();
+            List<Object> values = new ArrayList<>();
+            try (ResultSet result = connection.createStatement()
+                    .executeQuery("SELECT \"g\", " + nth + " FROM (VALUES " + rows + ") t(\"g\", \"x\") GROUP BY \"g\" ORDER BY \"g\"")) {
+                while (result.next()) {
+                    values.add(result.getObject(2));
+                }
+            }
+            return values;
+        }
+    }
+
+    @Test
+    void theNthValueOfEachGroupInTheOrderOfTheColumns() throws SQLException {
+        assertEquals(List.of(2, 7), nth(2, "(1, 1), (1, 2), (1, 3), (2, 7), (2, 8)"));
+    }
+
+    @Test
+    void aGroupWithFewerValuesGivesNull() throws SQLException {
+        List<Object> values = nth(5, "(1, 1), (1, 2), (1, 3), (1, 4)");
+        assertEquals(1, values.size());
+        assertNull(values.get(0));
+    }
+}

dialect/db/oracle/src/main/java/org/eclipse/daanse/sql/dialect/db/oracle/OracleDialect.java

@@ -237,9 +237,13 @@ public boolean supportsCreateOrReplaceTrigger() {
         return true;
     }
 
+    /**
+     * From Oracle 23 ({@code DROP TABLE IF EXISTS} came with 23ai); before, a drop of a missing table fails with
+     * ORA-00942. A dialect made without a connection knows no version and says no.
+     */
     @Override
     public boolean supportsDropTableIfExists() {
-        return false;
+        return dialectVersion.atLeast(23, 0);
     }
 
     @Override

dialect/db/oracle/src/test/java/org/eclipse/daanse/sql/dialect/db/oracle/OracleDropTableIfExistsTest.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation.
+ *
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *   SmartCity Jena - initial
+ */
+package org.eclipse.daanse.sql.dialect.db.oracle;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.eclipse.daanse.sql.dialect.api.DialectInitData;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Oracle drops a table only if it exists from version 23 on: before, the plain {@code DROP TABLE} of a missing table
+ * fails with ORA-00942, which aborted every CSV fixture of the ROLAP suite on Oracle Free 23ai (database run
+ * 18.10.2026).
+ */
+class OracleDropTableIfExistsTest {
+
+    private static OracleDialect dialect(int major) {
+        return new OracleDialect(DialectInitData.ansiDefaults().withVersion(major, 0));
+    }
+
+    @Test
+    void oracle23DropsATableIfItExists() {
+        assertThat(dialect(23).supportsDropTableIfExists()).isTrue();
+        assertThat(dialect(23).dropTable("S", "T", true)).isEqualTo("DROP TABLE IF EXISTS \"S\".\"T\"");
+    }
+
+    @Test
+    void oracle19AndADialectWithoutAVersionDropPlainly() {
+        assertThat(dialect(19).supportsDropTableIfExists()).isFalse();
+        assertThat(new OracleDialect().supportsDropTableIfExists()).isFalse();
+    }
+}