clickhouse-(09)- row_number() over (partition by)的几种实现方法

ClickHouse row_number() over (partition by)的几种实现方法

hive中有row_number() over (partition by)函数，可以一句SQL实现想要的排序，在ClickHouse中有很多种实现方式，本篇就介绍一下几种方法。

目录

1.row_number排序

2.row_number排序后取出rank=1的结果

3.特殊场景

1.row_number排序

HIVE中写法：

select number,
       row_number() over (partition by number order by time desc) as rank
  from table a
 GROUP BY number

ClickHouse写法：

select number,
       groupArray(time) AS arr_val,
       arrayEnumerate(arr_val) as row_number
  from (select distinct orderid as number,
               toDate(operatetime) as time
          from table
         order by time desc
         ) a
 GROUP BY number

2.row_number排序后取出rank=1的结果

hive写法：

select orderid
  from (select orderid,
               row_number() over(partition by orderid order by datachange_lasttime desc) as row_num
          from table
         where d = '${CurrentDate}'
         ) a
 where row_num = 1;

ClickHouse写法：

方法1：利用groupArray

select orderid, 
       groupArray(1)(datachange_lasttime) as dates
  from (select orderid, 
               datachange_lasttime
          from table
         ORDER BY orderid, datachange_lasttime desc
        ) a
 group by orderid

方法2：利用max函数实现倒序，如果正序使用min函数即可

select orderid,
       max(datachange_lasttime) as datachange_lasttime
  from table
 group by orderid

方法3：利用rowNumberInAllBlocks函数

select orderid, status
  from (select orderid, status, rowNumberInAllBlocks() as rank
          from (select orderid, status, datachange_lasttime
                  from table
                 order by orderid, datachange_lasttime desc
                 ) a
       ) b LIMIT 1 BY orderid

方法4：利用arrayEnumerate函数

select orderid
  from (select orderid,
               groupArray(datachange_lasttime) AS arr_val,
               arrayEnumerate(arr_val) as row_number
          from (select orderid, datachange_lasttime
                  from table
                 order by datachange_lasttime desc
                 ) a
         GROUP BY number
         ) b
 where row_number = 1

3.特殊场景
要求：

对于以下场景，需要按照orderid分组，按照日期倒序，取最新一条，若日期一致，则随机取一条作为结果即可

hive写法：

select orderid  from (select orderid,               status,               row_number() over(partition by orderid order by datachange_lasttime desc) as row_num          from table         where d = '${CurrentDate}'         ) as b where row_num = 1

ClickHouse写法：

通过上面的案例，我们很容易想到，把上面的结果作为一个子表，与原表进行关联，只是这样关联，随便举一个关联的写法：

select a.orderid as orderid_a, a.status as status  from olap_htlmaindb.tmp_ord_orders_status_s_pre a inner join (select orderid, groupArray(1)(datachange_lasttime) as dates               from (select orderid, datachange_lasttime                       from table                      ORDER BY orderid, datachange_lasttime desc                     ) a              group by orderid) b    on a.orderid = b.orderid   and cast(a.datachange_lasttime as String) = cast(b.dates [ 1 ] as String) 

这里我们是先把符合要求的orderid和时间取出来，再回去关联，取出需要的列，因为这些函数都有一个缺点是只能有partition by的字段和排序字段，不能有其他字段，所以要返回关联，所以上面四种方法，ininer join原表，都不能解决上面案例的问题。

这里就想到了LIMIT 1 BY这个方法，这个方法其实是最有效的，如下：

select orderid,        status,        datachange_lasttime  from table order by orderid, datachange_lasttime desc  LIMIT 1 BY orderid