我正在尝试添加一个新的列，它按日期显示行结果的排名（或序列）。
我写过：

SELECT 
@row_number:=(CASE
    WHEN @member_id = lh.member_id and lc.ladder_advocacy is not null 
        THEN @row_number + 1
    when @member_id = lh.member_id and lc.ladder_advocacy is null then "null"
    ELSE 1 /* there is an error here - i need it to return a 1 if not null, then 2 for the 2nd instance, etc */ 
END) AS rank_advocacy,
@member_id:=lh.member_id AS member_id,
lh.ladder_change,
lc.name,
lc.ladder_advocacy,
lc.ladder_elected,
lc.ladder_policy,
lc.ladder_engagement,
lc.ladder_newventure,
lc.ladder_collective,
lc.is_trigger
FROM
leenk_ladder_history AS lh
    LEFT JOIN
leeds_so.leenk_ladder_config AS lc ON lh.ladder_config_id = lc.id
WHERE
ladder_change = 1 AND trigger_active = 1
ORDER BY member_id, trigger_event_date DESC;

第4行有一个错误，我不知道如何修复它。对于第一个结果，我想返回1。对于第二个结果，我想返回@row\u number+1。第三个结果，@row\u number+2（etc）。
我如何做到这一点？