Question

我正在尝试这样做，以便下面的查询计算来自另一个具有用户名的表中的行。统计信息表中每个用户名有1行，但pvp表中有多行用户名。从pvptable加入的所有列都显示为null，并且kills＆amp;使用COUNT动态生成的死亡数。这是SQL查询。

SELECT 
  *,
  pvptable.username AS USER,
  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 = "Username" 
    AND pvptable.username = "Username" 
WHERE st.username = "Username" ;

Answer 1

您正在混合where和join条件：

如果要将一个表的字段与另一个表的字段相关联，请使用join
如果要过滤表格数据，请使用where

因此，要使您的查询有效：

SELECT 
    *, pvptable.username as user, 
    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 -- Add some other relations if you need them
WHERE 
    st.username="Username";

Answer 2

为你想要的每个属性做一个连接，然后使用COUNT（DISTINCT id）（假设它是pvp表的唯一键），如下所示： -

SELECT st.username as user, 
    COUNT(DISTINCT pvptable_kills.id) as kills,
    COUNT(DISTINCT pvptable_deaths.id) as deaths,
    ROUND(COUNT(DISTINCT pvptable_kills.id) / COUNT(DISTINCT pvptable_deaths.id), 2) AS kd  
FROM stats as st
LEFT OUTER JOIN pvp as pvptable_kills ON pvptable_kills.killer=st.Username
LEFT OUTER JOIN pvp as pvptable_deaths ON pvptable_deaths.username=st.Username
WHERE st.username="Username"
GROUP BY st.username

MySQL LEFT JOIN列为空

2 个答案: