[jOOQ/jOOQ#12910] Add DSL.emptyGroupingSet(): GroupField for explicit

empty grouping sets

This also fixes:

- [jOOQ/jOOQ#12908] Select:: loses GROUP BY clause

jOOQ/src/main/java/org/jooq/impl/EmptyGroupingSet.java

@@ -0,0 +1,59 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Other licenses:
+ * -----------------------------------------------------------------------------
+ * Commercial licenses for this work are available. These replace the above
+ * ASL 2.0 and offer limited warranties, support, maintenance, and commercial
+ * database integrations.
+ *
+ * For more information, please visit: http://www.jooq.org/licenses
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ */
+package org.jooq.impl;
+
+import static org.jooq.impl.SQLDataType.OTHER;
+
+import org.jooq.Context;
+
+/**
+ * @author Lukas Eder
+ */
+final class EmptyGroupingSet extends AbstractField<Object> implements QOM.EmptyGroupingSet {
+
+    static final EmptyGroupingSet INSTANCE = new EmptyGroupingSet();
+
+    private EmptyGroupingSet() {
+        super(DSL.name("()"), OTHER);
+    }
+
+    @Override
+    public final void accept(Context<?> ctx) {
+        ctx.sql("()");
+    }
+}