added query builder for orm

+// Copyright 2014 beego Author. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package orm
+
+type QueryBuilder interface {
+	Select(fields ...string) QueryWriter
+	From(tables ...string) QueryWriter
+	Where(cond string) QueryWriter
+	LimitOffset(limit int, offset int) QueryWriter
+	InnerJoin(table string) QueryWriter
+	LeftJoin(table string) QueryWriter
+	On(cond string) QueryWriter
+	And(cond string) QueryWriter
+	Or(cond string) QueryWriter
+	In(vals ...string) QueryWriter
+	Subquery(query string, rename string) string
+	String() string
+}

+// Copyright 2014 beego Author. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package orm
+
+import (
+	"fmt"
+	"strings"
+)
+
+type MySQLQueryBuilder struct {
+	QueryString []string
+}
+
+func (qw *MySQLQueryBuilder) Select(fields ...string) QueryWriter {
+	segment := fmt.Sprintf("SELECT %s", strings.Join(fields, ", "))
+	qw.QueryString = append(qw.QueryString, segment)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) From(tables ...string) QueryWriter {
+	segment := fmt.Sprintf("FROM %s", strings.Join(tables, ", "))
+	qw.QueryString = append(qw.QueryString, segment)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) Where(cond string) QueryWriter {
+	qw.QueryString = append(qw.QueryString, "WHERE "+cond)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) LimitOffset(limit int, offset int) QueryWriter {
+	qw.QueryString = append(qw.QueryString, fmt.Sprintf("LIMIT %d OFFSET %d", limit, offset))
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) InnerJoin(table string) QueryWriter {
+	qw.QueryString = append(qw.QueryString, "INNER JOIN "+table)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) LeftJoin(table string) QueryWriter {
+	qw.QueryString = append(qw.QueryString, "LEFT JOIN "+table)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) On(cond string) QueryWriter {
+	qw.QueryString = append(qw.QueryString, "ON "+cond)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) And(cond string) QueryWriter {
+	qw.QueryString = append(qw.QueryString, "AND "+cond)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) Or(cond string) QueryWriter {
+	qw.QueryString = append(qw.QueryString, "OR "+cond)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) In(vals ...string) QueryWriter {
+	segment := fmt.Sprintf("IN (%s)", strings.Join(vals, ", "))
+	qw.QueryString = append(qw.QueryString, segment)
+	return qw
+}
+
+func (qw *MySQLQueryBuilder) Subquery(sub string, alias string) string {
+	return fmt.Sprintf("(%s) AS %s", sub, alias)
+}
+
+func (qw *MySQLQueryBuilder) String() string {
+	return strings.Join(qw.QueryString, " ")
+}