Updated Druid_SQL_AST (markdown)

Druid_SQL_AST.md

@@ -11,6 +11,18 @@ public interface SQLExpr extends SQLObject {}
 public interface SQLStatement {}
 ```
 
+## 2.1 常用的SQLStatemment
+最常用的Statement当然是SELECT/UPDATE/DELETE/INSERT，他们分别是
+```java
+package com.alibaba.druid.sql.ast.statement;
+
+public class SQLSelectStatement {}
+public class SQLDeleteStatement {}
+public class SQLInsertStatement {}
+public class SQLUpdateStatement {}
+
+```
+
 # 3. 怎样产生AST
 
 ## 3.1 通过SQLUtils产生List<SQLStatement>