MySQL LEFT JOIN查询显示相同的值

Question

I am trying to make it so that this query below counts rows from another table that have the username, then deaths that have killer. There is 1 row per username in the stats table, but multiple rows with in the pvp table with the username. The deaths column is the same amount as the kills column for some reason, does anyone know why? Here is my query. Here is a sql fiddle I think I got it right idk how to use sqlfiddle: 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

To do it for everyone, use 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) and COUNT(bar) both count the number of rows in the resultset, you might as well say COUNT(*) in both cases.

You need to COUNT(*) FROM ... GROUP BY killer to get the number of killers, for example. Not sure what the table structure is to give you a full answer.