improved sql parser, bug fixed for parent is null

src/main/java/com/alibaba/druid/sql/ast/expr/SQLAggregateExpr.java

@@ -75,6 +75,13 @@ public class SQLAggregateExpr extends SQLExprImpl implements Serializable {
     public List<SQLExpr> getArguments() {
         return this.arguments;
     }
+    
+    public void addArgument(SQLExpr argument) {
+        if (argument != null) {
+            argument.setParent(this);
+        }
+        this.arguments.add(argument);
+    }
 
     public SQLOver getOver() {
         return over;

src/main/java/com/alibaba/druid/sql/ast/expr/SQLGroupingSetExpr.java

@@ -29,6 +29,13 @@ public class SQLGroupingSetExpr extends SQLExprImpl {
     public List<SQLExpr> getParameters() {
         return parameters;
     }
+    
+    public void addParameter(SQLExpr parameter) {
+        if (parameter != null) {
+            parameter.setParent(this);
+        }
+        this.parameters.add(parameter);
+    }
 
     @Override
     protected void accept0(SQLASTVisitor visitor) {

src/main/java/com/alibaba/druid/sql/ast/expr/SQLListExpr.java

@@ -29,6 +29,13 @@ public class SQLListExpr extends SQLExprImpl {
     public List<SQLExpr> getItems() {
         return items;
     }
+    
+    public void addItem(SQLExpr item) {
+        if (item != null) {
+            item.setParent(this);
+        }
+        this.items.add(item);
+    }
 
     @Override
     protected void accept0(SQLASTVisitor visitor) {