Question

我在MySQL中有表格，比如thoose：

troll_id     troll_family_id    troll_name    troll_birth_timestamp
--------------------------------------------------------------------
1            1                  Kamil         100000
2            1                  Bartek        200000
3            2                  Maciek        100000
4            2                  Andrzej       200000

troll_family_id    troll_family_name
------------------------------------------
1                  Trollowski
2                  Trollowicz

我想要only one the youngest Troll from every famillies

所以我想得到这样的行：

troll.troll_name
-----------------    
Bartek
Andrzej

我在数据库中有数百万的巨魔，我尝试过这样的查询：

SELECT troll.troll_name 
FROM troll 
GROUP BY troll.troll_family_id 
ORDER BY troll.troll_birth_timestamp DESC
LIMIT 0, 50

但它冻结了我对http的回应。所以我甚至不知道，这个查询是否正确。

Answer 1

试试这个：

SELECT a.troll_name 
FROM troll a 
INNER JOIN (SELECT troll_family_id, MAX(troll_birth_timestamp) AS troll_birth_timestamp 
            FROM troll 
            GROUP BY troll_family_id
           ) AS b on a.troll_family_id = b.troll_family_id AND a.troll_birth_timestamp = b.troll_birth_timestamp
LIMIT 0, 50

Answer 2

轻微更改现有查询。 ORDER BY在GROUP BY之后起作用：

SELECT troll_family_name, t.troll_name 
FROM
(
  SELECT troll_family_name, t.troll_name 
  FROM troll t
  JOIN troll_family tf ON tf.troll_family_id=t.troll_family_id
  ORDER BY t.troll_birth_timestamp DESC      
)temp
GROUP BY troll_family_name
LIMIT 0, 50

更多添加索引到tf.troll_family_id，t.troll_family_id

Answer 3

您可以在群组

上使用MAX功能

SELECT troll.troll_name, MAX(troll_birth_timestamp) AS troll_birth_timestamp
FROM troll 
GROUP BY troll.troll_family_id 
LIMIT 0, 50

如何在mysql非常大的表中制作更快的组？

3 个答案: