How do I use row_number() with partitioning and without ordering? - mysql

Question

My table looks like :
table_1
| Id | Num |
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
| 5 | 1 |
| 6 | 2 |
| 7 | 2 |
I want a row_number next to 'num' column, but as soon as the num changes it's value, the row_number resets.
I want my table to look like:
| Id | Num | row_num |
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 1 | 1 |
| 6 | 2 | 1 |
| 7 | 2 | 2 |

Answer 1

One way to get your desired output is to use lag and a conditional sum to flag when the number changes, then you can use row_number and partition by this flag:
with lagNum as (
select id, num, Lag(num) over(order by id) as v
from t
), changed as (
select id, num,
Sum(case when num = v then 0 else 1 end) over(order by id rows unbounded preceding) as v
from lagNum
)
select id, num, row_number() over(partition by v order by id) as row_num
from changed
Example Fiddle
This does require at least MySql 8 which added support for window functions

Answer 2

This is a type of gaps-and-islands problem. For this version, the simplest solution is probably to identify the islands using the difference of row numbers:
select t.*,
row_number() over (partition by seqnum - seqnum_2 order by id) as row_num
from (select t.*,
row_number() over (order by id) as seqnum,
row_number() over (partition by num order by id) as seqnum_2
from table_1 t
) t;
If you run the subquery, you will see how the difference identifies the "adjacent" values of num.
Note: If (as in your example) the ids are sequential with no gaps, you can simplify this to:
select t.*,
row_number() over (partition by id - seqnum_2 order by id) as row_num
from (select t.*,
row_number() over (partition by num order by id) as seqnum_2
from table_1 t
) t;