mysql - 在 Hive 中使用 case 语句填充前一个值

Question

我在下表中有一个列event，Hive如下所示。

Event

Sent
Sent
Open
Open
Click
Sent
Open
Signup
Sent
Open
Click

现在我想event使用 case 语句根据列中的值创建新列。

我想signup在事件列中的位置我希望该Previous_event列正好是preceding值。

我已经尝试过如下。

select event, 
       case when event = 'Sent' then 'No_event' 
            when event = 'Open' then 'Sent' 
            when event = 'Click' then 'Open'  
            else -1 
       end as Previous_event 
from table;

Result

Sent    No_event
Sent    No_event
Open    Sent
Open    Sent
Click   Open
Sent    No_event
Open    Sent
Signup  -1
Sent    No_event
Open    Sent
Click   Open

Expected result

Sent    No_event
Sent    No_event
Open    Sent
Open    Sent
Click   Open
Sent    No_event
Open    Sent
Signup  Open
Sent    No_event
Open    Sent
Click   Open

我怎样才能实现我想要的？

Answer 1

使用相关子查询。

根据OP 的评论，您可以尝试以下查询（对于 MySQL）：

select t1.event, 
       case when t1.event = 'Sent' then 'No_event' 
            when t1.event = 'Open' then 'Sent' 
            when t1.event = 'Click' then 'Open' 
            when t1.event = 'Signup' then (select t2.event 
                                           from table as t2 
                                           where t2.eventdate < t1.eventdate 
                                           order by t2.eventdate desc 
                                           limit 1)         
            else -1 
       end as Previous_event 
from table as t1;

Answer 2

以下是包含类似问题/解决方案的 URL：

Hive 访问上一行值

https://community.hortonworks.com/questions/39533/fill-null-with-previous-row-values-in-hive.html

SQL 将是：

select event, prev_event(event) as Previous_event from table;

UDF 的代码：

    import org.apache.hadoop.hive.ql.exec.UDF;

    public class cum_mul extends UDF  {
    private String prevValue = null;

    public String evaluate(String value) {

    switch(value) 
        { 
            case "Sent": 
        prevValue = "No_event";
                return "No_event"; 
            case "Open": 
        prevValue = "Sent";
                return "Sent";  
            case "Click": 
        prevValue = "Open";
                return "Open"; 
            default: 
                return prevValue; 
        } 
      }
}