Updated Druid_SQL_AST (markdown)

Druid_SQL_AST.md

@@ -114,6 +114,26 @@ class SQLWithSubqueryClause {
 }
 ```
 
+## 2.4
+```java
+interface SQLSelectQuery extends SQLObject {}
+
+class SQLSelectQueryBlock implements SQLSelectQuery {
+    List<SQLSelectItem> selectList;
+    SQLTableSource from;
+    SQLExprTableSource into;
+    SQLExpr where;
+    SQLSelectGroupByClause groupBy;
+    SQLOrderBy orderBy;
+    SQLLimit limit;
+}
+
+class SQLUnionQuery implements SQLSelectQuery {
+    SQLSelectQuery left;
+    SQLSelectQuery right;
+    SQLUnionOperator operator; // UNION/UNION_ALL/MINUS/INTERSECT
+}
+```
 
 # 3. 怎样产生AST