How to add conditional count based on mutiple columns - sql

Question

I'm trying to summarise a T-SQL output that looks a little like this:
+---------+---------+-----+-------+
| perf_no | section | row | seat |
+---------+---------+-----+-------+
| 7128 | 6 | A | 4 |
| 7128 | 6 | A | 5 |
| 7128 | 6 | A | 7 |
| 7128 | 6 | A | 9 |
| 7128 | 6 | A | 28 |
| 7129 | 6 | A | 29 |
| 7129 | 6 | A | 8 |
| 7129 | 6 | A | 9 |
| 7129 | 8 | A | 6 |
| 7129 | 8 | B | 3 |
| 7129 | 8 | B | 4 |
+---------+---------+-----+-------+
Comparing one row to the row(s) below, if the perf_no, section, and row values are the same, and the difference between the seat values is 1, then I want to consider them a group, and count the number of rows in that group.
To give you a real world example, these are seats in a theatre! I'm trying to summarise what seats are available.
Using the table above to illustrate:
rows 1 & 2 show that seats 4 & 5 in section 6, row 8 for performance 7128 are available. So that's 2 seats together
row 3 shows that 7 in sectino 6, row 8 for performance 7128 is available on its own. So that's a single seat (1)
rows 5 & 6 have the same section and row, and the seats are consecutive, but you can see the performance is different. So that's a single seat too.
So the output for the table above would look a little like...
(I've left in the spaces just so visually you can see the groupings more easily - obviously the final version will have none)
+---------+---------+----------+-------+
| perf_no | section | seat_row | total |
+---------+---------+----------+-------+
| 7128 | 6 | A | 2 |
| | | | |
| 7128 | 6 | A | 1 |
| 7128 | 6 | A | 1 |
| 7128 | 6 | A | 1 |
| 7129 | 6 | A | 1 |
| 7129 | 6 | A | 2 |
| | | | |
| 7129 | 6 | A | 1 |
| 7129 | 8 | B | 2 |
+---------+---------+----------+-------+
I've been trying to use some conditional case statements to not much avail. Any assistance very gratefully received!

Answer 1

This is a type of gaps-and-islands problem. You can generate a grouping by subtracting a sequence (generated by row_number()) from the seat:
select perf_no, section, row, count(*) as num_seats,
min(seat) as first_seat, max(seat) as last_seat
from (select t.*,
row_number() over (partition by perf_no, section, row order by seat) as seqnum
from t
) t
group by perf_no, section, row, (seat - seqnum);