refactor sql parser about the desc and explain parts of the code

src/main/java/com/alibaba/druid/sql/ast/statement/SQLDescribeStatement.java

@@ -15,14 +15,24 @@
  */
 package com.alibaba.druid.sql.ast.statement;
 
+import com.alibaba.druid.sql.ast.SQLExpr;
 import com.alibaba.druid.sql.ast.SQLName;
 import com.alibaba.druid.sql.ast.SQLStatementImpl;
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;
 
+import java.util.ArrayList;
+import java.util.List;
+
 public class SQLDescribeStatement extends SQLStatementImpl {
 
     protected SQLName object;
 
+    protected SQLName column;
+
+    // for odps
+    protected SQLObjectType objectType;
+    protected List<SQLExpr> partition = new ArrayList<SQLExpr>();
+
     public SQLName getObject() {
         return object;
     }
@@ -30,9 +40,40 @@ public class SQLDescribeStatement extends SQLStatementImpl {
     public void setObject(SQLName object) {
         this.object = object;
     }
-    
+
+    public SQLName getColumn() {
+        return column;
+    }
+
+    public void setColumn(SQLName column) {
+        if (column != null) {
+            column.setParent(this);
+        }
+        this.column = column;
+    }
+
+    public List<SQLExpr> getPartition() {
+        return partition;
+    }
+
+    public void setPartition(List<SQLExpr> partition) {
+        this.partition = partition;
+    }
+
+    public SQLObjectType getObjectType() {
+        return objectType;
+    }
+
+    public void setObjectType(SQLObjectType objectType) {
+        this.objectType = objectType;
+    }
+
     @Override
     protected void accept0(SQLASTVisitor visitor) {
-        throw new UnsupportedOperationException(this.getClass().getName());
+        if (visitor.visit(this)) {
+            acceptChild(visitor, object);
+            acceptChild(visitor, column);
+        }
+        visitor.endVisit(this);
     }
 }