CREATE TABLE table_56594 ("Driver" TEXT, "Constructor" TEXT, "Laps" FLOAT, "Time/Retired" TEXT, "Grid" FLOAT)

what's the highest lap that contains the driver of graham hill and a grid that is larger than 18?
SELECT MAX("Laps") FROM table_56594 WHERE "Driver" = 'graham hill' AND "Grid" > '18'
SELECT MAX("Laps") FROM "table_56594" WHERE "Driver" = 'graham hill' AND "Grid" > '18'