ranking语句中的用户定义变量

Question

在这个问题上我会得到任何帮助的。我已经花了好几个小时没有任何真正的解决办法。我有一个SQL

SELECT to_place, rank
FROM
  (SELECT g1.to_place as to_place, g1.pcount as pcount, 
                @rank := IF(@current_to_place = g1.to_place, @rank + 1, 1) AS rank,
                @current_to_place := g1.to_place 
  FROM 
          (select 
          to_place, count(*) as pcount
          from temp_workflows
          group by to_place
          order by to_place,pcount desc) g1
  ORDER BY g1.to_place, g1.pcount DESC) ranked

在表g1中，我对我的数据进行分组，以找到to_place最常见的情况，然后我想按升序排列这些情况（这样我以后就可以在每个to_place类别中选择最常见的前3个）。

问题是用户定义的变量是不可预测的(@rank有时总是1)，这可能与这样一个事实有关，即在一个语句中，我不应该引用同一个变量(current_to_place)。我读了很多关于使用单独的语句等，但我可以找到一种方法，以一种不同的方式来写我的语句。我如何在别处定义@current_to_place以使结果相同？事先谢谢你的帮助。

Answer 1

我认为您应该测试pcount以获得rank,并且应该初始化变量

DROP TABLE IF EXISTS T;
CREATE TABLE T
(to_place int);

insert into t values (1),(2),(2),(3),(3),(3);
SELECT to_place, rank
FROM
  (
  SELECT g1.to_place as to_place, g1.pcount as pcount, 
                @rank := IF(@current_to_place <> pcount, @rank + 1, 1) AS rank,
                @current_to_place := pcount 
  FROM 
          (select 
          to_place, count(*) as pcount
          from t
          group by to_place
          order by to_place,pcount desc) g1
 cross join(select @rank:=0,@current_to_place:=0) r
  ORDER BY g1.pcount DESC
  )
   ranked

+----------+------+
| to_place | rank |
+----------+------+
|        3 |    1 |
|        2 |    2 |
|        1 |    3 |
+----------+------+
3 rows in set (0.016 sec)