mysql - 如何获取最新记录

Question

我有一个看起来像这样的表:

+--------+-------------+----------+---------+
| PK(id) | FK(user_id) | ctime    | comment |
+--------+-------------+----------+---------+
| 1      | 1           | 20170101 | "Haha"  |
+--------+-------------+----------+---------+
| 2      | 2           | 20170102 | "Nope"  |
+--------+-------------+----------+---------+
| 3      | 2           | 20170104 | "Geez"  |
+--------+-------------+----------+---------+
| 4      | 1           | 20170110 | "Gone"  |
+--------+-------------+----------+---------+

我希望检索每个 FK(user_id) 的最新记录，如下所示:

+--------+-------------+----------+---------+
| PK(id) | FK(user_id) | ctime    | comment |
+--------+-------------+----------+---------+
| 3      | 2           | 20170104 | "Geez"  |
+--------+-------------+----------+---------+
| 4      | 1           | 20170110 | "Gone"  |
+--------+-------------+----------+---------+

所以我尝试了 SELECT DISTINCT T.* FROM my_table AS T和 SELECT DISTINCT T.user_id FROM my_table AS T；但是，它们不起作用!

我试过 GROUP BY 语句，如下所示，

SELECT T.* FROM my_table AS T GROUP BY `user_id` DESC

它确实如我所料完美地工作。所以我开始研究如何将 SQL 翻译成 Django!

首先，我尝试使用 RawSQL:

from django.db.models.expressions import RawSQL

def _select_latest_rows(model_class, target_column_name, query_set=None):
    query_set = query_set or model_class.objects
    table_name = model_class._meta.db_table
    raw_sql = '''
    SELECT * FROM %s GROUP BY %s
    '''
    return query_set.annotate(val=RawSQL(raw_sql, (table_name, target_column_name,)))

每当我调用此函数时，Django 都会弹出以下错误。

django.db.utils.ProgrammingError: (1064, "You have an error in your SQL syntax
blah blah blah...

然后我检查了我的查询字符串:

print(_select_latest_rows(model_class, target_column_name, query_set).query)

它返回:

SELECT T.`id`, T.`user_id`, T.`ctime`, T.`comment`, (SELECT * FROM my_table GROUP BY user_id) AS `val` FROM my_table

好吧，我不得不说我对 SQL 语法不是很熟悉，到目前为止我还没有弄清楚这个查询字符串中的错误所在:(

我必须在模型的管理器实例中调用 raw() 函数，如 this answer 中所建议的那样？

model_class.objects.raw('SELECT T.* FROM my_table AS T GROUP BY `user_id` DESC LIMIT 10 OFFSET 0')

或者也许我不应该在开始时使用 GROUP BY 语句？

Answer 1

最简单的方法是:

select * 
from my_table t
where ctime = (select max(ctime) from my_table where user_id = t.user_id);

但是，如果 user_id 有多个相同的日期，您也可以在内部查询中使用 limit 子句

select * 
from my_table t
where id = (select id 
            from my_table 
            where  user_id = t.user_id
            order by ctime desc 
            LIMIT 1);