trigger or check constraint insert values based upon existence of others

For simplicity (and since it captures the essence of my problem), suppose I have a table with data.

CREATE TABLE my_practice_table
(

my_id       NUMBER(12), 

field_2     VARCHAR2(20),

field_3     VARCHAR2(20)

);

I want to define a new constraint on the table that has the following logic:

For any new record x inserted into my_practice_table,

if (x.field_2 is null)

   if [there exists a record y in the table such that 

   i.) y.my_id == x.my_id 

   AND

   ii.) y.field_2 is not null
    ]

       --do not insert x into the table

   else
      --there exists no such y in the table, so we insert x

else (x.field_2 is not null)

 if [there exists a record y in the table such that
    i.) y.my_id == x.my_id
    AND
    ii.) y.field_2 is null
   ]
     --do not insert x into the table
else 
     --there exists no such y in the table, so we insert x

Example: Suppose my_practice_table had the following data
table1: [1 'hi' 'bye' ]
[2 'hello' 'adios']
[1 'hi' 'seeya']

The statement:
INSERT INTO table1
(my_id, field_2, field_3)
VALUES
(1, null, ‘ciao’);

won’t work because there is already a record in table1 with my_id equal to 1 and field_2 not equal to null.

Example: Suppose my_practice_table had the following data:
table2: [1 'hi' 'bye' ]
[2 null 'adios']
[1 'hi' 'seeya']

The statement:
INSERT INTO table2
(my_id, field_2, field_3)
VALUES
(2, ‘hola’, ‘this won’t work’);

won’t work because there is already a record in table2 with my_id equal to 2 and field_2 equal to null.

My question is what is the way to go about this in Oracle syntax. The algorithm is simple enough, but I’m just not comfortable enough with Oracle syntax to get the job done.
Do I use a BEFORE INSERT Trigger? How do I scan for values in other fields of a particular row once I found a match on the field my_id? Is there a ‘break’ statement in Oracle that I can do to speed this up?
Any help or guidance to the right place to look for a possible solution would be appreciated. Thanks