CREATE TABLE table_203_193 (id DECIMAL, "ship name" TEXT, "in service" DECIMAL, "project number" TEXT, "type" TEXT, "class" TEXT, "comments" TEXT)

what is the difference in years between the first ship put into service and the last ?
SELECT MAX("in service") - MIN("in service") FROM table_203_193
SELECT MAX("in service") - MIN("in service") FROM "table_203_193"