mysql tæller hvor mange gange et felt linker til et andet

select ref, count(distinct id) from table group by ref;

Dette vil give dig en optælling af distinkt id ved ref.

select ref, count(*) from table group by ref;

Dette vil give dig antallet af poster pr. ref.

EDIT:

Prøv dette for at få det output, du ønsker.

select t.*, m.counter
from table t 
join (
   select ref, count(distinct id) as counter
   from table group by ref
) m on t.ref = m.ref

Eksempel på SQLFiddle:http://sqlfiddle.com/#!9/2b93c/2