fix: use Function and SelectItem

Signed-off-by: Andreas Reichel <andreas@manticore-projects.com>

Author: manticore-projects

src/main/java/net/sf/jsqlparser/statement/piped/PivotPipeOperator.java

@@ -23,7 +23,7 @@ public PivotPipeOperator(Function aggregateExpression, Column inputColumn,
         this.alias = alias;
     }
 
-    public Expression getAggregateExpression() {
+    public Function getAggregateExpression() {
         return aggregateExpression;
     }
 

src/main/java/net/sf/jsqlparser/statement/select/Pivot.java

@@ -99,6 +99,12 @@ public Pivot withForColumns(ExpressionList<Column> forColumns) {
         return this;
     }
 
+    public Pivot addForColumn(Column... forColumns) {
+        ExpressionList<Column> forColumnsList = new ExpressionList<>(forColumns);
+        this.setForColumns(forColumnsList);
+        return this;
+    }
+
     public Pivot withSingleInItems(List<SelectItem<?>> singleInItems) {
         this.setSingleInItems(singleInItems);
         return this;