各类型Builder：相当于层层嵌套

func (self *SelectBuilder) Where(condition string) *WhereBuilder {
	where := newWhereBuilder(condition)
	self.where = where
	where.prev = self
	return where
}
func (self *SelectBuilder) OrderBy(column string) *OrderByBuilder {
	orderby := newOrderByBuilder(column)
	self.orderby = orderby
	orderby.prev = self
	return orderby
}
func (self *SelectBuilder) Limit(offset, n int) *LimitBuilder {
	limit := newLimitBuilder(offset, n)
	limit.prev = self
	self.limit = limit
	return limit
}

Where、OrderBy、Limit有没有都不影响调用ToString();Where里的And和Or有没有都不影响调用ToString()