SQL query: Documentation

CREATE FUNCTION vavalkusas.get_reservation_item_price(
reservation_item_id INT
) RETURNS INT RETURN SELECT if( sp_interval = "day", SUM( (
(
TO_DAYS( LEAST( s_date_end, r_date_last ) ) - TO_DAYS( GREATEST( r_date_from, s_date_start ) ) ) +1 ) * sp_price
), (
ri_amount * ri_price
)
) AS total_price
FROM reservations
INNER JOIN reservation_items ON ri_reservationsid = r_id
INNER JOIN products ON ri_productsid = p_id
INNER JOIN product_groups ON if( p_type = "house", p_house_public_productgroupsid, p_productgroupsid ) = pg_id
INNER JOIN season_prices ON sp_productgroupsid = pg_id
INNER JOIN seasons ON s_id = sp_seasonsid
AND r_date_from <= s_date_end
AND r_date_last >= s_date_start
WHERE ri_id = @reservation_item_id
GROUP BY r_id

MySQL said: Documentation
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT        if (sp_interval = "day"
                    ,SUM ((
                        (
                            TO_DAYS( LEAST( s' at line 3