LAST_VALUE() AND FIRST_VALUE() best example

Scenario:-
Consider a Phone Log table as below. It records all phone numbers that we dial in a given day.

SOURCE_PHONE_NUMBER	DESTINATION_PHONE_NUMBER	CALL_START_DATETIME
1234	4567	01/07/2011 10:00
1234	2345	01/07/2011 11:00
1234	3456	01/07/2011 12:00
1234	3456	01/07/2011 13:00
1234	4567	01/07/2011 15:00
1222	7890	01/07/2011 10:00
1222	7680	01/07/2011 12:00
1222	2345	01/07/2011 13:00

Please provide an SQL query to display the source_phone_number and a flag where the flag needs to be set to Y if first called number and last called number are the same and N if the first called number and last called number are different.

Desired Output:-

Source Number	Is_Match
1222	N
1234	Y

Solution:-

SELECT source_number, 
       CASE 
              WHEN first_val=last_val THEN 'Y' 
              ELSE 'N' 
       END AS is_match 
FROM   

( 
SELECT   source_phone_number AS source_number,
First_value(destination_phone_number) over (PARTITION BY source_phone_number ORDER BY call_start_datetime) AS first_val,
Last_value(destination_phone_number) over (PARTITION BY source_phone_number ORDER BY call_start_datetime)  AS last_val,
Row_number() over (PARTITION BY source_phone_number ORDER BY call_start_datetime)                          AS id )
WHERE  id=1 
FROM   phone_log ) WHERE id=1.

Can test query's in the blow site:-                                                                                   

http://www.sqlfiddle.com/#!4/198cd/1