Question

我正在努力寻找名字为'杰克'的玩家的身份以及他们玩过的游戏数量：

Select p.id, count(*) as numOfGamePlayed
from player p, game g
where p.name = 'Jack' and p.id = g.id
group by p.id;

问题是，这只会列出至少玩过一场比赛的球员名称杰克。我怎样才能列出那些没玩过任何游戏的人？

编辑：对于那些玩家，numOfGamePlayed必须为0.如果我这样做

Select p.id, count(*) as numOfGamePlayed
from player p LEFT JOIN game g ON p.id = g.id
where p.name = 'Jack'
group by p.id;

那些没有玩任何游戏的玩家仍会将numOfGamePlayed显示为1

Answer 1

停止为初学者使用隐式联接。无论如何，这是一个坏习惯。然后你可以这样做：

Select p.id, count(g.id) as numOfGamePlayed
from player p
LEFT JOIN game g
 ON p.id = g.id
where p.name = 'Jack'
group by p.id;

Answer 2

你需要玩家和游戏之间的外部联接

SELECT p.ID, 
  COUNT(*) AS numOfGamePlayed
FROM player p
LEFT OUTER JOIN game g
ON p.id = g.id
WHERE p.name = 'Jack'
GROUP BY p.ID;

Answer 3

SELECT p.id, count(*) as numOfGamePlayed
FROM player p
LEFT JOIN game g ON g.id = p.id
WHERE p.name = 'Jack'
--AND g.id IS NULL
GROUP BY p.id;

Answer 4

你必须做count（game.PlayerId）而不是count（*）

See SQL Fiddle demo here

select p.PNAME, g.gameplayed
from player p 
left outer join game g
ON p.pid = g.pid;

Select p.pid, count(g.pid) as numOfGamePlayed
from player p 
LEFT JOIN game g 
ON p.pid = g.pid
where p.pname = 'Jack'
group by p.pid;

具有零关系的SQL计数记录

4 个答案: