mybatis 集合

1.分组统计：

SELECT user_id,company_name,count(*) FROM t_message GROUP BY user_id;

2.mybatis使用group by返回多个值，包含count数据，该怎么接返回值：

https://blog.csdn.net/weixin_43276786/article/details/94547839

3.mybaits plus 在满足条件的记录中，随机取20条，且只查指定字段

        List<Teacher> list = teacherService.list(
                new QueryWrapper<Teacher>()
                        .eq("status", "available")
                        .select("name", "gender", "avatar_url")
                        .last("ORDER BY RAND() LIMIT 0,"+number));

4.mybatis 时间判断

        <if test="signDateStart != null &amp; signDateStart !=''">
            AND unix_timestamp(sign_date) &gt;= unix_timestamp(#{signDateStart})
        </if>
        <if test= "signDateEnd != null  &amp; signDateEnd !=''">
            AND unix_timestamp(sign_date) &lt;= unix_timestamp(#{signDateEnd})
        </if>