CREATE TABLE table_54812 ("Driver" TEXT, "Constructor" TEXT, "Laps" FLOAT, "Time/Retired" TEXT, "Grid" FLOAT)

What is the mean number of laps when time/retired was spun off and the driver was Nick Heidfeld?
SELECT AVG("Laps") FROM table_54812 WHERE "Time/Retired" = 'spun off' AND "Driver" = 'nick heidfeld'
SELECT AVG("Laps") FROM "table_54812" WHERE "Driver" = 'nick heidfeld' AND "Time/Retired" = 'spun off'