SQL Server 2017

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CREATE TABLE tablename (
	placename VARCHAR(10),
	value REAL,
	id INTEGER
);
INSERT INTO tablename (placename,value,id) VALUES 
 ('A',1.1,1),
 ('A',1.05,2),
 ('A',2,3),
 ('A',3,4),
 ('B',2,1),
 ('B',5,2),
 ('B',2.1,3),
 ('B',7,4),
 ('C',1,1),
 ('C',2,2),
 ('C',3,3),
 ('C',17,4),
 ('C',17.11,5),
 ('C',17.6,6);

14 rows affected

select distinct t.placename, t.id 
from tablename t inner join tablename tt 
on t.placename = tt.placename  
where t.id <> tt.id and tt.value between t.value - 0.5 and t.value + 0.5
order by t.placename, t.id


placename
id


A
1

A
2

B
1

B
3

C
4

C
5

C
6


select distinct t.placename, t.id 
from tablename t 
where exists (
  select 1 from tablename
  where placename = t.placename and id <> t.id 
  and value between t.value - 0.5 and t.value + 0.5  
)
order by t.placename, t.id


placename
id


A
1

A
2

B
1

B
3

C
4

C
5

C
6