如何在外鍵不返回任何內容時在MySQL中選擇記錄？

Question

SELECT videos.id, videos.game_id, videos.xbox360, videos.ps3, videos.pc, 
    videos.wii, videos.other, videos.thumbnail, videos.vid_info, videos.sdvid, 
    videos.hdvid, UNIX_TIMESTAMP(videos.date_added) , game_data.name, 
    AVG(video_ratings.rating) 
FROM videos, game_data, video_ratings 
WHERE videos.game_id = game_data.id 
    AND videos.id = video_ratings.video_id 
GROUP BY videos.id, video_ratings.video_id 
ORDER BY videos.date_added DESC LIMIT 10; 

我運行此查詢從三個表video_ratings，game_data，視頻中提取數據...

現在我面臨的問題是，結果只顯示已被評爲（或者是在視頻表video_ratings），因爲AND videos.id = video_ratings.video_id ...

有什麼辦法，我可以選擇所有的視頻數據，結果表明AVG(video_ratings.rating)爲空，如果爲這些影片的收視率不存在video_ratings表（或者說沒有的視頻已被評分，因此結果必須顯示10行AVG(video_ratings.rating)列空）...

感謝

Answer 1

呀，只是用左外內而不是內部連接（這是你在做什麼）。我也建議你改用JOIN語法。它更清晰：

SELECT v.id, v.game_id, v.xbox360, v.ps3, v.pc, v.wii, 
v.other, v.thumbnail, v.vid_info, v.sdvid, v.hdvid, 
UNIX_TIMESTAMP(v.date_added), gd.name, AVG(vg.rating) 
FROM videos v 
LEFT JOIN game_data gd ON gd.id = v.game_id 
LEFT JOIN video_ratings vr ON v.id = vr.video_id 
GROUP BY videos.id, video_ratings.video_id 
ORDER BY videos.date_added DESC LIMIT 10