backported some code from lang_de branch

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

src/main/java/de/srsoftware/widerhall/data/Database.java

@@ -91,6 +91,7 @@ public class Database {
      */
     public class Request{
 
+        private String groupBy = null;
         private final StringBuilder sql; // buffer the sql to be built
         private final HashMap<String, List<Object>> where = new HashMap<>(); // buffer condition statements for select
         private final HashMap<String,Object> values = new HashMap<>(); // buffer values for insert/update statements
@@ -128,6 +129,10 @@ public class Database {
             }
         }
 
+        private void applyGrouping(){
+            if (groupBy != null && !groupBy.isBlank()) sql.append(" GROUP BY ").append(groupBy.trim());
+        }
+
         private void applySorting(){
             if (!sortFields.isEmpty()) sql.append(" ORDER BY ").append(String.join(", ",sortFields));
         }
@@ -148,6 +153,7 @@ public class Database {
             var args = new ArrayList<>();
             applyValues(args);
             applyConditions(args);
+            applyGrouping();
             applySorting();
             if (additionalArgs != null) {
                 for (Object arg : additionalArgs) args.add(arg);
@@ -187,6 +193,10 @@ public class Database {
             }
         }
 
+        public Request groupBy(String column) {
+            groupBy = column;
+            return this;
+        }
 
         public void run() throws SQLException {
             compile().run();