Question

Consider a table

results (id, key, value) where key is the Primary key.

Sample data:

id | key      | value
-----------------------
abc| source-1 | 20
abc| source-2 | 30
abc| source-3 | 2 
abc| source-4 | 10
def| source-5 | 1 
ghi| source-6 | 25
jkl| source-5 | 13

I would like to return only those records which have a single entry for a given id. So output should be

id | key      | value
------------------------
def| source-5 | 1
ghi| source-6 | 25
jkl| source-5 | 13

Please advise.

Answer 1

One way is to use GROUP BY and HAVING to produce a derived table with the desired ids and then JOIN to it:

select results.*
from results
join (
    select id
    from results
    group by id
    having count(*) = 1
) as dt on results.id = dt.id

You could also use an IN if you don’t like derived tables:

select *
from results
where id in (
    select id
    from results
    group by id
    having count(*) = 1
)