各組獲取狀態由N個最後一行在MySQL

Question

這裏被簡化表（sqlfiddle）：

id group status 
-- ------ ------- 
1 group1 success 
2 group1 fail 
3 group1 success 

4 group2 success 

5 group3 fail 
6 group3 success 
7 group3 success 
8 group3 success 

我需要得到有關結果由N個最後一行的失敗對每個組的信息。 這樣的（N = 3）：

group has_failures 
------ ------------ 
group1 1 
group2 0 
group3 0 

獲得團體是很容易的：

-- Select last row from each group: 
SELECT a.id, a.group, a.status 
FROM log a 
INNER JOIN(
    select max(i.id) as max_id 
    from log i 
    group by i.group 
) as b on a.id = b.max_id; 

和失敗過：

-- Select fail status by last 3 rows: 
select count(g.status) > 0 as has_failures 
from (
     SELECT a.group, a.status 
     FROM log a 
     WHERE a.group = 'group3' 
     ORDER BY id DESC 
     LIMIT 3 
    ) as g 
where g.status = 'fail'; 

如何應該是這兩個查詢合併，或可能更簡單的方式存在

Answer 1

這裏有一個選項，使用user defined variables來爲每個組建立一個行號。然後你可以使用conditional aggregation：

select max(t.id) id, 
    t.group, 
    max(case when t.rank = 1 then t.status end) status, 
    sum(case when t.status = 'fail' then 1 else 0 end) hasfailures 
from (
    select *, 
    case l.group 
     when @curGroup 
     then @curRow := @curRow + 1 
     else @curRow := 1 AND @curGroup := l.group 
    end + 1 AS rank 
    from `log` l cross join (select @curRow:=0, @curGroup:='') c 
    order by l.group, id desc 
) t 
where rank <= 3 
group by t.group 

• SQL Fiddle Demo