我如何将 COUNT 与 JOIN SQL 3 表一起使用

Question

我有 3 张桌子。

第一个表是带有列的战斗：

--battleId(primary)--gameId(foreign)--prize--

第二张表是带有列的游戏：

--gameId(primary)--gameName--

第三张表是 BattleParticipants 列：

--battleParticipantsId(primary)--userId(foreign)--battleId(foreign)--score--

首先我编写了这个查询来加入战斗和游戏并获取battleId、prize和gameName数据：

SELECT
    battles.battleId
  , battles.prize
  , games.gameName 
FROM
  battles 
JOIN 
  games 
ON 
  battles.gameId = games.gameId

此查询返回每个战斗的battleId,prize,gameName数据。

我想扩展这个查询以获得每场战斗的参与者数量（这意味着 BattleParticipant 表中的行，Battles.battleId = BattleParticipants.battleId ） 通过以某种方式将其与 BattleParticipants 表结合起来。

所以结果应该是： BattleId，奖品，游戏名称，Battle 参与者人数

示例表和结果：

战斗：

 --battleId(primary)--gameId(foreign)--prize--
      asd123               1               100
      asd1234              2               200
      asd1235              1               300

游戏：

  --gameId(primary)--gameName--
           1             Some Game
           2             Another Game

战斗参与者：

   --battleParticipantsId(primary)--userId(foreign)--battleId(foreign)--score--
                 1                     qwedqweqw       asd123             100
                 2                     qweqwe         asd1234             200
                 3                     qweadgjhg      asd123              400
                 4                       asd           asd123             500

预期结果：

[ RowDataPacket {
    battleId: 'asd123',
    prize: 100,
    gameName: 'Some Game',
    numberOfParticipantInBattle: 3 },
  RowDataPacket {
    battleId: 'asd1234',
    prize: 200,
    gameName: 'Some Game',
    numberOfParticipantInBattle: 1 },
  RowDataPacket {
    battleId: 'asd1235',
    prize: 300,
    gameName: 'Some Game',
    numberOfParticipantInBattle: 0 }
 ]

如果我尝试@Gordon Linoff 给出的答案，这就是我得到的结果：

[ RowDataPacket {
    battleId: 'asd123',
    prize: 100,
    gameName: 'Some Game',
    numberOfParticipantInBattle: 3 },
  RowDataPacket {
    battleId: 'asd1234',
    prize: 200,
    gameName: 'Some Game',
    numberOfParticipantInBattle: 1 },
  RowDataPacket {
    battleId: 'asd1235',
    prize: 300,
    gameName: 'Some Game',
    numberOfParticipantInBattle: 0 },
  RowDataPacket {
    battleId: 'asd123',
    prize: 100,
    gameName: 'Another Game',
    numberOfParticipantInBattle: 3 },
  RowDataPacket {
    battleId: 'asd1234',
    prize: 200,
    gameName: 'Another Game',
    numberOfParticipantInBattle: 1 },
  RowDataPacket {
    battleId: 'asd1235',
    prize: 300,
    gameName: 'Another Game',
    numberOfParticipantInBattle: 0 } ]

有什么方法可以删除重复项还是我应该手动删除

Answer 1

我认为你只需要一个额外的JOIN 和GROUP BY：

SELECT b.battleId, b.prize, g.gameName,
       COUNT(*) as numberOfParticipantInBattle
FROM battles b JOIN
     games g
     ON b.gameId = b.gameId JOIN
     BattleParticipants bp
     ON b.battleId = bp.battleId
GROUP BY b.battleId, b.prize, g.gameName;

如果某些战斗可能没有参与者，而您想将他们包括在内，请使用LEFT JOIN：

SELECT b.battleId, b.prize, g.gameName,
       COUNT(bp.battleId) as numberOfParticipantInBattle
FROM battles b JOIN
     games g
     ON b.gameId = b.gameId LEFT JOIN
     BattleParticipants bp
     ON b.battleId = bp.battleId
GROUP BY b.battleId, b.prize, g.gameName;