Mybatis-PageHelper自定义查询COUNT

使Mybatis-PageHelper用分页插件，可写自定义查询COUNT的SQL语句，在列表查询的id后面加上后缀_COUNT，如下：

<select id="selectLeftjoin" resultType="com.github.pagehelper.model.User">
    select a.id,b.name,a.py from user a
    left join user b on a.id = b.id
    order by a.id
</select>
<select id="selectLeftjoin_COUNT" resultType="Long">
    select count(distinct a.id) from user a
    left join user b on a.id = b.id
</select>

详细请参考：https://github.com/pagehelper/Mybatis-PageHelper/blob/master/wikis/zh/Changelog.md#504---2017-08-01