MySQL 8.0

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select version();


version()


8.0.13


create table existing (id int unsigned not null)
insert into existing values (3),(5),(6),(7),(8),(19),(20),(21),(30)
set @last=0
CREATE TEMPORARY TABLE v (gap_from int unsigned, gap_to int unsigned, next int unsigned, PRIMARY KEY(gap_from, gap_to))
IGNORE SELECT IF(@last=id, 0, @last) as gap_from,
   IF(@last=id, 0, id-1) as gap_to,
   @last:=id+1 as next
FROM existing ORDER BY id
select gap_from,gap_to from v where NOT (gap_from=0 AND gap_to=0)


gap_from
gap_to


0
2

4
4

9
18

22
29