Which Diameter (km) has a Name of alma-merghen planitia, and a Year named smaller than 1997?
CREATE TABLE "planitia" (
  "name" text,
  "latitude" text,
  "longitude" text,
  "diameter_km" real,
  "year_named" real
);
SELECT MAX("diameter_km") FROM "planitia" WHERE "name"='alma-merghen planitia' AND "year_named"<1997;