Add support of PostgreSQL-style MODE()

h2/src/docsrc/help/help.csv

@@ -3417,8 +3417,8 @@ MEDIAN(X)
 "
 
 "Functions (Aggregate)","MODE","
-MODE( value )
-[ ORDER BY { expression [ ASC | DESC ] } ]
+{ MODE( value ) [ ORDER BY expression [ ASC | DESC ] ] }
+    | { MODE() WITHIN GROUP(ORDER BY expression [ ASC | DESC ]) }
 [ FILTER ( WHERE expression ) ]
 ","
 Returns the value that occurs with the greatest frequency.
@@ -3434,6 +3434,7 @@ Aggregates are only allowed in select statements.
 ","
 MODE(X)
 MODE(X ORDER BY X)
+MODE() WITHIN GROUP(ORDER BY X)
 "
 
 "Functions (Numeric)","ABS","

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

@@ -2968,26 +2968,32 @@ public class Parser {
             break;
         }
         case MODE: {
-            Expression expr = readExpression();
-            r = new Aggregate(aggregateType, expr, currentSelect, false);
-            if (readIf(ORDER)) {
+            if (readIf(CLOSE_PAREN)) {
+                read("WITHIN");
+                read(GROUP);
+                read(OPEN_PAREN);
+                read(ORDER);
                 read("BY");
-                Expression expr2 = readExpression();
-                String sql = expr.getSQL();
-                if (!sql.equals(expr2.getSQL())) {
-                    parseIndex = lastParseIndex;
-                    if (expectedList != null) {
-                        expectedList.clear();
-                        expectedList.add(sql);
+                Expression expr = readExpression();
+                r = new Aggregate(AggregateType.MODE, expr, currentSelect, false);
+                setModeAggOrder(r, expr);
+            } else {
+                Expression expr = readExpression();
+                r = new Aggregate(aggregateType, expr, currentSelect, false);
+                if (readIf(ORDER)) {
+                    read("BY");
+                    Expression expr2 = readExpression();
+                    String sql = expr.getSQL();
+                    if (!sql.equals(expr2.getSQL())) {
+                        parseIndex = lastParseIndex;
+                        if (expectedList != null) {
+                            expectedList.clear();
+                            expectedList.add(sql);
+                        }
+                        throw getSyntaxError();
                     }
-                    throw getSyntaxError();
+                    setModeAggOrder(r, expr);
                 }
-                ArrayList<SelectOrderBy> orderList = new ArrayList<>(1);
-                SelectOrderBy order = new SelectOrderBy();
-                order.expression = expr;
-                order.sortType = parseSimpleSortType();
-                orderList.add(order);
-                r.setOrderByList(orderList);
             }
             break;
         }
@@ -3003,6 +3009,15 @@ public class Parser {
         return r;
     }
 
+    private void setModeAggOrder(Aggregate r, Expression expr) {
+        ArrayList<SelectOrderBy> orderList = new ArrayList<>(1);
+        SelectOrderBy order = new SelectOrderBy();
+        order.expression = expr;
+        order.sortType = parseSimpleSortType();
+        orderList.add(order);
+        r.setOrderByList(orderList);
+    }
+
     private ArrayList<SelectOrderBy> parseSimpleOrderList() {
         ArrayList<SelectOrderBy> orderList = Utils.newSmallArrayList();
         do {

h2/src/test/org/h2/test/scripts/functions/aggregate/mode.sql

@@ -42,5 +42,14 @@ SELECT MODE(V ORDER BY V DESC) FROM TEST;
 SELECT MODE(V ORDER BY V + 1) FROM TEST;
 > exception SYNTAX_ERROR_2
 
+SELECT MODE() WITHIN GROUP(ORDER BY V) FROM TEST;
+>> 1
+
+SELECT MODE() WITHIN GROUP(ORDER BY V ASC) FROM TEST;
+>> 1
+
+SELECT MODE() WITHIN GROUP(ORDER BY V DESC) FROM TEST;
+>> 3
+
 DROP TABLE TEST;
 > ok