[sql] How to count the number of occurrences of a character in an Oracle varchar value?

Here you go:

select length('123-345-566') - length(replace('123-345-566','-',null)) 
from dual;

Technically, if the string you want to check contains only the character you want to count, the above query will return NULL; the following query will give the correct answer in all cases:

select coalesce(length('123-345-566') - length(replace('123-345-566','-',null)), length('123-345-566'), 0) 
from dual;

The final 0 in coalesce catches the case where you're counting in an empty string (i.e. NULL, because length(NULL) = NULL in ORACLE).