<sql></sql>标签是干嘛的

<sql id="Base_Column_List">
  id, emp_id, emp_name, org_id, org_name, corporate_name, project_name, position, project_duty,
  project_describe, project_achievement, start_time, end_time, remark,explains, create_time,
  update_time, update_emp_id, update_emp_name
</sql>
<select id="selectByPrimaryKey" parameterType="java.lang.String" resultMap="BaseResultMap">
  select
  <include refid="Base_Column_List" />
  from t_project_experience
  where id = #{id,jdbcType=VARCHAR}
</select>
上面的sql标签是相当于一个*因为