SQL : how to find gaps in a specific range of numbers? - sql

Question

I want to find gaps in a numeric column (not the entire column), but in a specific range.
For example :
My column :
1
2
5
6
8
10
18
19
20
I want to specify a specific range in which my SQL query would look for gaps. For example, I want gaps in the range [15,20]. In this example, the gaps are : 15,16,17.
I built a query that retrieves gaps but not in case the gaps are at the beginning of my range
SELECT cur_value + 1 AS start_gap, next_value - 1 AS end_gap
FROM (
SELECT col AS cur_value, LEAD (col) OVER (ORDER BY col) AS next_value
FROM table
--WHERE col BETWEEN 200 AND 300
)
WHERE next_value - cur_value > 1
ORDER BY start_gap;
How can I do that ?
N.B : Performance is very important in my case. I deal with tons of rows.

Answer 1

I think the simplest method might be:
SELECT cur_value + 1 AS start_gap, next_value - 1 AS end_gap
FROM (SELECT col AS cur_value, LEAD (col) OVER (ORDER BY col) AS next_value
FROM (SELECT col
FROM table
WHERE col BETWEEN 200 AND 300
UNION ALL
SELECT 200-1 FROM DUAL
UNION ALL
SELECT 300+1 FROM DUAL
) t
) t
WHERE next_value - cur_value > 1
ORDER BY start_gap;
Note: This will work on arbitrarily long ranges.

Answer 2

You can simply use hierarchical connect by to produce the list of numbers in the given range and then check to see if the value doesn't exist in the table.
Assuming your range is 15 to 20, use NOT IN:
select * from (
select level - 1 + 15 col
from dual
connect by level <= 20 - 15
) where col not in (select col from your_table);
Demo
Similarly, NOT EXISTS:
select * from (
select level - 1 + 15 col
from dual
connect by level <= 20 - 15
) t where not exists (select 1 from your_table where col = t.col);
Demo 2