How to select gaps in my sqlite database - sqlite

Question

I'm trying to write down a select to show me all the items that do not exist in my database. Say I have a table named TBL795 with a column named NRBEM which could not have any gaps in it.
Should be this way:
Those numbers are in sequence.
If they looked like this:
they would be wrong, because some items have not being inserted into the table.
In a table with thousand of items it would be very difficult to find out if there are any gaps and which are the missing items.
One solution would be this:
CREATE TABLE TESTE (
NRBEM VARCHAR(15))
feed it with a command like this:
INSERT INTO TESTE
WITH RECURSIVE
cnt(NRBEM) AS (VALUES(1) UNION ALL SELECT NRBEM+1 FROM cnt WHERE NRBEM <100000)
SELECT NRBEM FROM cnt A
and running this select
SELECT A.NRBEM FROM TESTE A LEFT JOIN TBL795 B
ON A.NRBEM = B.NRBEM
WHERE B.NRBEM IS NULL
I can see all the items that are missing in my table.
Since the command:
WITH RECURSIVE
cnt(NRBEM) AS (VALUES(1) UNION ALL SELECT NRBEM+1 FROM cnt WHERE NRBEM <100000)
SELECT NRBEM FROM cnt
create a virtual table I would like to run a select like this:
SELECT NRBEM FROM (
WITH RECURSIVE
cnt(NRBEM) AS (VALUES(1) UNION ALL SELECT NRBEM+1 FROM cnt WHERE NRBEM <100000)
SELECT NRBEM FROM cnt ) A LEFT JOIN TBL795 B
ON A.NRBEM = B.NRBEM
But this does not work.
This way:
SELECT X FROM (
WITH RECURSIVE
cnt(X) AS (VALUES(1) UNION ALL SELECT X+1 FROM cnt WHERE X <100000)
SELECT X FROM cnt ) A LEFT JOIN TBL795 B
ON A.X = B.NRBEM
it works, but does not select the right items.
So, how could I write this select?

Answer 1

It would be possible to use an outer join and filter out matches, but using set operations is simpler:
WITH RECURSIVE CNT(NRBEM) AS (...)
SELECT NRBEM
FROM CNT
WHERE NRBEM NOT IN (SELECT NRBEM
FROM tbl795);

Answer 2

I found out what I was doing wrong.
If I cast nrbem as number the select works.
SELECT A.X FROM (
WITH RECURSIVE
cnt(X) AS (VALUES(1) UNION ALL SELECT X+1 FROM cnt WHERE X <100000)
SELECT X FROM cnt ) A LEFT JOIN ( SELECT CAST( NRBEM AS NUMBER ) AS NRBEM FROM TBL795 ) B
ON A.X = B.NRBEM
WHERE B.NRBEM IS NULL
If I want to check the range from item 2400 to 2700 to see if there are any gaps I can do this:
SELECT A.X FROM (
WITH RECURSIVE
cnt(X) AS (VALUES(2400) UNION ALL SELECT X+1 FROM cnt WHERE X < (2700))
SELECT X FROM cnt ) A LEFT JOIN ( SELECT CAST( NRBEM AS NUMBER ) AS NRBEM FROM TBL795 WHERE NRBEM >= 2400 and nrbem <= 2700 ) B
ON A.X = B.NRBEM
WHERE B.NRBEM IS NULL
LIMIT ( 2700 - 2400 + 1 )