What's the default value of an union in avro Idl?
okay, I found the answer (from the apache user guide).
The default type of an union will be always it's first value. So, here the default types for the above examples would be NULL, STRING, NULL and NULL. And the default values would be the ones those assigned to the union.
So, in the above unions only the first and second unions are syntactically correct and I think all other (3 and 4) unions are syntactically wrong because defaultValues assigned to them doesn't match the union's defaultType.