Oracle Dual Table Default Value

You can do something like this :

SELECT *
FROM vehicle_register
WHERE (vehicle_type = @param OR @param is null)

You are using parameterized queries and not just a query build by string concatination in the frontend I hope.

Assuming that sql_id has a unique or primary key constraint, you can do this with FULL JOIN (or LEFT JOIN if there are no new "inserts" in table B, or just JOIN if there are no inserts and no deletions):

SELECT 
    COALESCE(a.sql_id,b.sql_id) AS sql_id,
    CASE WHEN a.operation = b.operation THEN 'matched' 
         WHEN a.operation <> b.operation THEN 'value changed'
         WHEN b.operation IS NULL THEN 'a deleted'
         WHEN a.operation IS NULL THEN 'b inserted' 
    END AS status,
    a.operation AS a_operation,
    b.operation AS b_operation
FROM 
  a FULL JOIN b
      ON a.sql_id = b.sql_id ;