ERROR at line 1:
ORA-00001: unique constraint (user.constraint_name) violated

    SELECT tran_id
    FROM TRANS
    WHERE tran_id='DUP7927';
    
    IF (ID_EXISTS) THEN do something
    ELSE do something else
    END IF

DECLARE
  id integer;
  ...
BEGIN
  SELECT tran_id into id FROM TRANS WHERE tran_id='DUP7927';
  <do something>
EXCEPTION
  WHEN NO_DATA_FOUND THEN 
   <do other thing>
END;

OPEN yourcursor;
FETCH yourcursor INTO local_variable;

IF yourcursor%FOUND Then
   <do_someting>
ELSE
   <do_other_thing>
END IF;

CLOSE yourcursor;

create table Pk_test as select rownum as PK from dual connect by level < 100000;

alter table pk_test add primary key(PK);

DECLARE
  id NUMBER ;
BEGIN
select 1 into id from dual where exists ( select 1 from pk_test where PK = 55555);
DBMS_OUTPUT.PUT_LINE('EXISTS');
EXCEPTION
  WHEN NO_DATA_FOUND THEN
    DBMS_OUTPUT.PUT_LINE('NOT EXISTS');
END;
/

Execution Plan
----------------------------------------------------------
Plan hash value: 1906259202

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |              |     1 |       |     3   (0)| 00:00:01 |
|*  1 |  FILTER            |              |       |       |            |          |
|   2 |   FAST DUAL        |              |     1 |       |     2   (0)| 00:00:01 |
|*  3 |   INDEX UNIQUE SCAN| SYS_C0039342 |     1 |    13 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter( EXISTS (SELECT /*+ */ 0 FROM "PK_TEST" "PK_TEST" WHERE
              "PK"=55555))
   3 - access("PK"=55555)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        216  bytes sent via SQL*Net to client
        247  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

DECLARE
  v_id_exists BOOLEAN := TRUE;
  v_tran_id   trans.tran_id%type;
BEGIN

  BEGIN
    SELECT tran_id
    INTO   v_tran_id 
    FROM   TRANS 
    WHERE   tran_id= 'DUP7927';

  EXCEPTION
    WHEN no_data_found then
      v_id_exists := false;
  END;

  IF (id_exists) THEN do something
  ELSE do something else
  END IF
END;

  BEGIN
    SELECT tran_id
    INTO   v_tran_id 
    FROM   TRANS 
    WHERE   tran_id= 'DUP7927';

  EXCEPTION
    WHEN no_data_found THEN
      v_id_exists := false;
    WHEN too_many_rows THEN
      null;
  END;