มีนักว่ายน้ำกี่คนที่ไม่ได้ว่ายน้ำ ?ไม่จำเป็นต้องใช้โต๊ะทั้งหมด
how many swimmers did not swim ?It is not neccessary to use all the tables.
 table_204_886:  id [ number]  "rank" [ number]  "swimmer" [ text]  "country" [ text]  "time" [ text]  "note" [ text] 
SELECT COUNT("swimmer") FROM table_204_886 WHERE "rank" IS NULL