refactor sql parser.

src/main/java/com/alibaba/druid/sql/ast/SQLLimit.java

@@ -0,0 +1,53 @@
+package com.alibaba.druid.sql.ast;
+
+import com.alibaba.druid.sql.dialect.mysql.visitor.MySqlASTVisitor;
+import com.alibaba.druid.sql.visitor.SQLASTVisitor;
+
+/**
+ * Created by wenshao on 16/9/25.
+ */
+public class SQLLimit extends SQLObjectImpl {
+
+    public SQLLimit() {
+
+    }
+
+    public SQLLimit(SQLExpr rowCount) {
+        this.setRowCount(rowCount);
+    }
+
+    private SQLExpr rowCount;
+    private SQLExpr offset;
+
+    public SQLExpr getRowCount() {
+        return rowCount;
+    }
+
+    public void setRowCount(SQLExpr rowCount) {
+        if (rowCount != null) {
+            rowCount.setParent(this);
+        }
+        this.rowCount = rowCount;
+    }
+
+    public SQLExpr getOffset() {
+        return offset;
+    }
+
+    public void setOffset(SQLExpr offset) {
+        if (offset != null) {
+            offset.setParent(this);
+        }
+        this.offset = offset;
+    }
+
+    @Override
+    protected void accept0(SQLASTVisitor visitor) {
+        if (visitor.visit(this)) {
+            acceptChild(visitor, offset);
+            acceptChild(visitor, rowCount);
+        }
+        visitor.endVisit(this);
+    }
+
+}