什麼mysql查詢比較好？

Question

我有2個mysql查詢，檢索相同的數據？，你認爲哪個更好用，爲什麼？考慮到標準和更好的代碼等。對不起，如果這是一個愚蠢的問題，但我是一隻好奇的貓！所以這裏有雲：

查詢1：

SELECT * 
FROM 
((
    SELECT u.username, u.picture, m.id, m.user_note, m.reply_id, m.reply_name, m.dt 
    FROM relationships r 
    JOIN notes m ON m.user_id = r.leader 
    JOIN user u ON r.leader = u.user_id 
    WHERE r.listener ='2' 
) 
UNION ALL 
(
    SELECT u.username, u.picture, m.id, m.user_note, m.reply_id, m.reply_name, m.dt 
    FROM notes m 
    JOIN user u ON m.user_id = u.user_id 
    WHERE u.user_id ='2' 
)) d 
WHERE d.dt < '2010-09-20_131830' 
ORDER BY d.dt DESC 

QUERY 2：

SELECT u.username, u.picture, m.id, m.user_note, m.reply_id, m.reply_name, m.dt 
FROM notes m 
INNER JOIN user u 
ON m.user_id = u.user_id 
WHERE (m.user_id = '2' 
OR m.user_id IN (
    SELECT r.leader 
    FROM relationships r 
    WHERE r.listener ='2')) 
AND dt < '2010-09-20_131830' 
ORDER BY dt DESC 

Answer 1

我找到UNION ALL版本更容易理解（這可能是我offcourse），但我把它改寫如下。

將AND m.dt < '2010-09-20_131830添加到每個部分應該會給你更好的性能。
您可以運行它幾次，並驗證它是否有所作爲。

SELECT u.username 
     , u.picture 
     , m.id 
     , m.user_note 
     , m.reply_id 
     , m.reply_name 
     , m.dt 
FROM relationships r 
     INNER JOIN notes m ON m.user_id = r.leader 
     INNER JOIN user u ON r.leader = u.user_id 
WHERE r.listener ='2' 
     AND m.dt < '2010-09-20_131830' 
UNION ALL 
SELECT u.username 
     , u.picture 
     , m.id 
     , m.user_note 
     , m.reply_id 
     , m.reply_name 
     , m.dt 
FROM notes m 
     INNER JOIN user u ON m.user_id = u.user_id 
WHERE u.user_id ='2' 
     AND m.dt < '2010-09-20_131830' 
ORDER BY m.dt DESC