mariadb-10.3.3中增加了用户定义的聚合存储函数
mysql可以执行聚合函数，但不能在sql中执行。他们需要一个udf（共享库实现）

没有自定义项是可以做到的，我知道有两种方法。第一种方法使用两个select和一个join，第一个select获取值和排名，第二个select获取计数，然后将它们合并。第二种方法使用json函数在一个select中获取所有内容。它们都有点长，但它们工作起来很快。
解决方案#1（两个选择和一个连接，一个获得计数，一个获得排名）

SELECT  x.group_field, 
        avg(
            if( 
                x.rank - y.vol/2 BETWEEN 0 AND 1, 
                value_field, 
                null
            )
        ) as median
FROM (
    SELECT  group_field, value_field, 
            @r:= IF(@current=group_field, @r+1, 1) as rank, 
            @current:=group_field
    FROM (
        SELECT group_field, value_field
        FROM table_name
        ORDER BY group_field, value_field
    ) z, (SELECT @r:=0, @current:='') v
) x, (
    SELECT group_field, count(*) as vol 
    FROM table_name
    GROUP BY group_field
) y WHERE x.group_field = y.group_field
GROUP BY x.group_field;

解决方案#2（使用json对象存储计数并避免连接）

SELECT group_field, 
    avg(
        if(
            rank - json_extract(@vols, path)/2 BETWEEN 0 AND 1,
            value_field,
            null
        )
    ) as median
FROM (
    SELECT group_field, value_field, path, 
        @rnk := if(@curr = group_field, @rnk+1, 1) as rank,
        @vols := json_set(
            @vols, 
            path, 
            coalesce(json_extract(@vols, path), 0) + 1
        ) as vols,
        @curr := group_field
    FROM (
        SELECT p.group_field, p.value_field, concat('$.', p.group_field) as path
        FROM table_name
        JOIN (SELECT @curr:='', @rnk:=1, @vols:=json_object()) v
        ORDER BY group_field, value_field DESC
    ) z
) y GROUP BY group_field;