MySQL分级/梯形图系统

我有两张桌子：

*tbl_比赛*

id
contestant_1
contestant_2
winner

*tbl_选手*

id
name

目标是获得每位参赛者的获胜百分比。也就是说，计算每个参赛者赢了多少场比赛，然后除以每个参赛者参加的比赛总数。这目前正在运行，但似乎很麻烦：

SELECT all_matches.contestant_id, num_matches, num_wins, 
    (num_wins / num_matches * 100.0) AS win_percent FROM ( 

        // get total number of wins for each contestant

        SELECT contestants.id AS contestant_id, 
        COUNT(matches.winner) AS num_wins 
        FROM contestants 
        LEFT JOIN matches 
        ON matches.winner = contestants.id 
        GROUP BY matches.winner 
    ) all_matches 

    // join total number of wins to total number of matches played

    JOIN ( 
        SELECT contestant_id, COUNT(contestant_id) AS num_matches FROM ( 

            // get list of all matches played by all contestants

            SELECT contestants.id AS contestant_id 
            FROM matches 
            JOIN contestants ON contestants.id = matches.contestant_1 
            UNION ALL 
            SELECT contestants.id AS contestant_id 
            FROM matches 
            JOIN contestants ON contestants.id = matches.contestant_2 
        ) all_m 
        GROUP BY contestant_id 
    ) all_matches 
    ON all_matches.contestant_id = all_matches.contestant_id 
    ORDER BY win_percent DESC

---

我觉得这类事情以前一定做过，我正在寻找一些帮助来优化这一点，或者寻找一个链接到一个更好的人已经做过的事情

我会尝试这种方法：

SELECT 
    contestants.id AS contestant_id,
    COUNT(*) AS num_matches,
    SUM( CASE WHEN matches.winner = contestants.id THEN 1 ELSE 0 END ) 
        AS num_wins,
    SUM( CASE WHEN matches.winner = contestants.id THEN 1 ELSE 0 END ) 
        / COUNT(*) * 100 AS win_percent 
FROM matches 
JOIN contestants 
ON contestants.id IN( matches.contestant_1, matches.contestant_2 ) 
GROUP BY contestants.id

---

考虑提供适当的DDL（和/或SqLFIDLE）以及期望的结果集-1：没有解释计划，没有创建表语句，没有索引细节，（在较小程度上，没有对非规范化模式设计的解释，没有理由选择每一个参赛者）很好。非常感谢。