MySQL LEFT JOIN查詢顯示相同的值

Question

我正在嘗試使它下面的查詢計數來自具有用戶名的另一個表中的行，然后計算具有殺手級的死亡。 stats表中的每個用戶名都有1行，而pvp表中的每個用戶名都包含多行。 由於某種原因，死亡人數欄與殺人人數欄相同，有人知道為什么嗎？ 這是我的查詢。 這是一個sql小提琴，我想我對idk的理解是如何使用sqlfiddle的： http ://sqlfiddle.com/#!2/b793b/1

SELECT 
    *,
    COUNT(pvptable.killer) as kills,   
    COUNT(pvptable.username) as deaths,
    ROUND(COUNT(pvptable.killer) / COUNT(pvptable.username), 2) as kd
FROM
    stats as st
    LEFT JOIN pvp as pvptable ON pvptable.killer=st.username
WHERE
    st.username="Username";

Answer 1

SELECT st.*,
       SUM(pvp.killer = st.username) AS kills,
       SUM(pvp.username = st.username) as deaths
FROM stats AS st
LEFT JOIN pvp ON pvp.username = st.username OR pvp.killer = st.username
WHERE st.username = "Username"

DEMO

為此，請使用GROUP BY ：

SELECT st.*,
       SUM(pvp.killer = st.username) AS kills,
       SUM(pvp.username = st.username) as deaths
FROM stats AS st
LEFT JOIN pvp ON pvp.username = st.username OR pvp.killer = st.username
GROUP BY st.username

DEMO

Answer 2

COUNT(foo)和COUNT(bar)都計算結果集中的行數，在這兩種情況下您都可以說COUNT(*) 。

例如，您需要COUNT(*) FROM ... GROUP BY killer以獲得COUNT(*) FROM ... GROUP BY killer的數量。 不確定表的結構將為您提供完整的答案。