MySQL 分組後取前幾條

利用group_concat和substring_index實現，能很好的利用索引，適合大資料。

select 

*from
yourtable
where
id in (select 
substring_index(group_concat(id
order by column_2 desc),
',',
1) id
from
yourtable
group by column_1)
order by column_2 desc;

由於在子查詢中，in (group_concat())會放棄索引使用，如下：

select * from yourtable where id in(

select 
group_concat(id)
from
yourtable
where
id in (select 
substring_index(group_concat(id
order by column_2 desc),
',',
8) id
from
yourtable
group by column_1)
order by column_2 desc
)

如果還是要一條語句，好吧。

select * from yourtable where find_in_set(id,(

select 
group_concat(id) id
from
yourtable
where
id in (select 
substring_index(group_concat(id
order by column_2 desc),
',',
8) id
from
yourtable
group by column_1)
order by column_2 desc
))

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