query to find the consecutive numbers

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Data Setup

create table t(c number);

insert into t values(6);

insert into t values(10);

insert into t values(11);

insert into t values(12);

insert into t values(57);

insert into t values(77);

insert into t values(98);

insert into t values(99);

insert into t values(100);

insert into t values(120);

insert into t values(125);

commit;

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Method 1

select c 

from (

    select c, 

           lag(c,1) over(order by c) lag,

           lead(c,1) over(order by c) lead

    from t)

where c=lag+1 or c=lead-1;

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Method 2

select c from (

select  c,count(1) over(partition by c-rownum) g

from t ) where g > 1;

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Method 3

select c.c

from t c, t prev

where c.c = prev.c +1

union

select c.c

from t c, t next

where c.c = next.c -1;

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