How to sum based on field value (mysql)

I got this table named tb_stock

ID AMOUNT FLAG LAST_AMOUNT
1   5     N    0                
2   0     Y    8                
3   0     Y    7                
4   6     N    0

Right now开发者_运维百科 i want to sum the value based on the FLAG value. For example if the FLAG values is N, it will sum from the AMOUNT field and if the FLAG value is Y, it will sum from the LAST_AMOUNT field.

So the total amount would be 26 (5 + 8 + 7 + 6).

Is this can be achieve with just use mysql or it will involve php as well?

Thanks.

SELECT SUM(CASE WHEN FLAG = 'Y' THEN LAST_AMOUNT ELSE AMOUNT END) AS TOTAL
FROM tb_stock

this is an example using IF statement

SELECT SUM(IF(`flag`='y', last_amount, amount))
FROM example_table