Ttyjukl

I need to show the running win/loss streak per row in a query, so given the table below, the query should return the "expected" column. I've tried some approaches with window functions, but no success.

create table matches (player text, dt date,  is_winner boolean, expected integer )

insert into matches values

('A', '2019-01-01', TRUE, 0),

('A', '2019-01-03', TRUE, 1),

('A', '2019-01-04', TRUE, 2),

('A', '2019-01-09', FALSE, 0),

('A', '2019-01-10', FALSE, -1),

('A', '2019-01-15', TRUE, 0);

player  dt          is_winner   expected

A       2019-01-01  true        0

A       2019-01-03  true        1

A       2019-01-04  true        2

A       2019-01-09  false       0

A       2019-01-10  false       -1

A       2019-01-15  true        0

The logic is:

1. Resets to 0 when winning after a loss, or losing after a win.


2. Increments after a win, but not if it's case 1.


3. Decrements after a loss, but not if it's case 1.

Any insights on how to tackle this are welcome. My last resort would be a function with a loop called by every row.

I've done this in stages using CTEs so that you can see how it's done as the queries progress. Each CTE adds a column in the output in order to show you progress.

It's pretty much self-documenting with the CTE names, to be honest.

with lags as (

  select player,

         dt,

         is_winner,

         lag(is_winner) OVER (partition by player ORDER BY dt ASC) as prev_is_winner,

         expected

  from matches

),

     group_changes as (

       select lags.*,

              case

                when prev_is_winner <> is_winner or prev_is_winner is null

                  then 1

                else 0

                end as is_new_group

       from lags

     ),

     groups_numbered as (

       select *,

              sum(is_new_group)

                  over (partition by player order by dt, is_winner desc) as streak_group

       from group_changes

     ),

     expected_in_groups as (

       select groups_numbered.*,

              row_number()

                  over (partition by player,streak_group

                    order by dt asc, streak_group asc) - 1 as expected_unsigned

       from groups_numbered

     )

select expected_in_groups.*, case when is_winner = 't' then expected_unsigned else expected_unsigned * -1 end as actual

from expected_in_groups

order by player asc, dt asc;

DB Fiddle Link (I added an extra row just to make sure it was working at a certain point)

Basically:

1. 
   lags CTE: use LAG() to get the previous result relative to the current row.


2. 
   group_changes CTE: Detect whether the previous streak, whether win or loss, has ended


3. 
   groups_numbered CTE: Give each streak a number


4. 
   expected_in_groups CTE: Number the rows in the group


5. Final select: negate the loss streaks

.

+--------+------------+-----------+----------------+----------+--------------+--------------+-------------------+--------+

| player |     dt     | is_winner | prev_is_winner | expected | is_new_group | streak_group | expected_unsigned | actual |

+--------+------------+-----------+----------------+----------+--------------+--------------+-------------------+--------+

| A      | 2019-01-01 | t         |                |        0 |            1 |            1 |                 0 |      0 |

| A      | 2019-01-03 | t         | t              |        1 |            0 |            1 |                 1 |      1 |

| A      | 2019-01-04 | t         | t              |        2 |            0 |            1 |                 2 |      2 |

| A      | 2019-01-09 | f         | t              |        0 |            1 |            2 |                 0 |      0 |

| A      | 2019-01-10 | f         | f              |       -1 |            0 |            2 |                 1 |     -1 |

| A      | 2019-01-11 | f         | f              |       -2 |            0 |            2 |                 2 |     -2 |

| A      | 2019-01-15 | t         | f              |        0 |            1 |            3 |                 0 |      0 |

+--------+------------+-----------+----------------+----------+--------------+--------------+-------------------+--------+