SQL Difference between two row in group by

Question

I have a table records of store id, processing batch id and start time as follows:

|store_id | batch_id | process_start_time |
| A       | 1        | 10                 |
| B       | 1        | 40                 |
| C       | 1        | 30                 |
| A       | 2        | 400                |
| B       | 2        | 800                |
| C       | 2        | 600                |
| A       | 3        | 10                 |
| B       | 3        | 80                 |
| C       | 3        | 90                 |

Here, rows needed to be grouped by batch_id and time_taken is difference of process_start_time of store A and store C .

So, the expected result would be:

batch_id | time_taken
1        | 20
2        | 200
3        | 80

I tried to do something like:

select batch_id, ((select process_start_time from records where store_id = 'C') - (select process_start_time from records where store_id = 'A')) as time_taken 
from records group by batch_id;

But couldn't figure out to select specific rows in that particular group.

Thank you for looking into!

Update: the process_start_time column not necessarily max for store C

Answer 1

You seem to want conditional aggregation and arithmetic:

select batch_id,
       (max(case when store_id = 'C' then process_start_time end) -
        min(case when store_id = 'A' then process_start_time end)
       ) as diff
from records
group by batch_id;

Answer 2

You can try a self join.

SELECT r1.batch_id,
       r1.process_start_time - r2.process_start_time time_taken
       FROM records r1
            INNER JOIN records r2
                       ON r1.batch_id = r2.batch_id
       WHERE r1.store_id = 'C'
             AND r2.store_id = 'A';

Answer 3

Here's another answer. This is using two instances of the records table and we link them up with where clauses and exists as follows:

select a.batch_id,
       c.process_start_time - a.process_start_time as time_taken
from   records a,
       records c
where  a.store_id = 'A'
and    c.store_id = 'C'
and    exists (
select 1
from   records x
where  x.batch_id = a.batch_id
and    x.batch_id = c.batch_id
);

Answer 4

SELECT DISTINCT 
    store_a.batch_id,
    store_c.process_start_time - store_a.process_start_time AS 'time_taken'
    FROM records store_a  
    INNER JOIN records store_c
    ON store_a.batch_id = store_c.batch_id 
       AND store_c.store_id = 'C' 
       AND  store_a.store_id = 'A'