MySQL返回一些行3次

Question

我有这个问题：

SELECT w.windate, w.wintime, w.field, w.htname, w.vtname, w.plateumpire, 
       w.fieldumpire1, w.fieldumpire2, w.fieldumpire3, p.pteamname, p.teamcoach, 
       p.pteamdivision, d.originalid 
    FROM sportsdb_wins AS w, playerteams AS p, sportsdb_divs AS d 
    WHERE p.teamcoach='$coachid' 
        AND p.pteamname IN (w.htname, w.vtname) 
        AND p.forteam='$teamid'

并且至少有一行返回3次！为什么？请帮忙我是SQL新手。 在PHP中使用它...

Answer 1

必须加入表格时使用JOIN。您当前的问题是所谓的笛卡尔积。

Answer 2

您没有为sportsdb_divs表定义连接条件，因此您将获得这些结果的交叉产品。如果您使用显式连接（您应该在所有SQL代码中执行此操作），这将更加明显。

SELECT w.windate, w.wintime, w.field, w.htname, w.vtname, w.plateumpire, 
       w.fieldumpire1, w.fieldumpire2, w.fieldumpire3, p.pteamname, p.teamcoach, 
       p.pteamdivision, d.originalid 
    FROM playerteams AS p
        INNER JOIN sportsdb_wins AS w
            ON p.teamname in (w.htname, w.vtname)
        INNER JOIN sportsdb_divs AS d 
            ON ?????
    WHERE p.teamcoach='$coachid' 
        AND p.forteam='$teamid'