CREATE TABLE table_203_270 (id NUMBER, "year" NUMBER, "competition" CLOB, "venue" CLOB, "position" CLOB, "event" CLOB, "notes" CLOB)

which four competitions are the only competitions that occurred once ?
SELECT "competition" FROM table_203_270 GROUP BY "competition" HAVING COUNT(*) = 1
SELECT "competition" FROM "table_203_270" GROUP BY "competition" HAVING COUNT(*) = 1