Question

我的表格如下：

    let tabBarController : UITabBarController = self.window?.rootViewController as! UITabBarController;
    tabBarController.selectedIndex = 0

    let navigationController  = tabBarController.selectedViewController as! UINavigationController
    let controllers = navigationController.viewControllers // will give array
    if controllers.count > 0 {
        if let viewC = controllers[0] as? DesiredViewController {
       // do desired work
       }
   }

我希望按照以下+--------+-------------+----------+---------+ | PK(id) | FK(user_id) | ctime | comment | +--------+-------------+----------+---------+ | 1 | 1 | 20170101 | "Haha" | +--------+-------------+----------+---------+ | 2 | 2 | 20170102 | "Nope" | +--------+-------------+----------+---------+ | 3 | 2 | 20170104 | "Geez" | +--------+-------------+----------+---------+ | 4 | 1 | 20170110 | "Gone" | +--------+-------------+----------+---------+检索最新记录：

FK(user_id)

所以我试过+--------+-------------+----------+---------+ | PK(id) | FK(user_id) | ctime | comment | +--------+-------------+----------+---------+ | 3 | 2 | 20170104 | "Geez" | +--------+-------------+----------+---------+ | 4 | 1 | 20170110 | "Gone" | +--------+-------------+----------+---------+ 和SELECT DISTINCT T.* FROM my_table AS T; 但是，他们不会工作！

我尝试了GROUP BY语句，如下所示，

SELECT DISTINCT T.user_id FROM my_table AS T

它确实像我期望的那样完美地工作。所以我开始弄清楚如何将SQL翻译成Django！

首先，我尝试使用RawSQL：

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

每当我调用此函数时，Django都会弹出以下错误。

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,)))

然后我检查了我的查询字符串：

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

它返回：

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

好吧，我不得不说我对SQL语法并不熟悉，到目前为止，我还没有弄清楚错误存在于此查询字符串中的位置：（

我必须在this answer中建议的模型管理器实例中调用SELECT T.`id`, T.`user_id`, T.`ctime`, T.`comment`, (SELECT * FROM my_table GROUP BY user_id) AS `val` FROM my_table函数吗？

raw()

或许我不应该在开头使用GROUP BY语句？

Answer 1

最简单明了的方法是：

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

但是，如果limit

多个相同日期，您也可以在内部查询中使用user_id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);

Answer 2

的Oracle / PostgreSQL的：

尝试使用以下SQL，它将满足您的要求。

select id,user_id,ctime,comment
from  (select   id,user_id,ctime,comment
                rank() over(partition by user_id order by ctime desc) rn
        from your_table_name
      ) s
where rn = 1;

如何获得最新记录

2 个答案: