在MySQL中获取不同计数的计数

Question

我有两个表，一个是团队的，一个是玩家的，我想找出的是总人数表，换句话说，我想统计拥有2个成员的团队总数，即所有团队的总数有3个成员，等等

这是数据库结构。

（侧边栏问题：我在这里是个新手：是否有更好的方法发布SQL？）

CREATE  TABLE `formsfiles`.`Teams` (
  `ID` INT NOT NULL AUTO_INCREMENT ,
  `Name` VARCHAR(45) NULL ,
  PRIMARY KEY (`ID`) );


INSERT INTO `Teams` (`Name`) VALUES ('Sharks');
INSERT INTO `Teams` (`Name`) VALUES ('Jets');
INSERT INTO `Teams` (`Name`) VALUES ('Fish');
INSERT INTO `Teams` (`Name`) VALUES ('Dodgers');


CREATE  TABLE `Players` (
  `ID` INT NOT NULL AUTO_INCREMENT ,
  `Name` VARCHAR(45) NULL ,
  `Team_ID` INT NULL ,
  PRIMARY KEY (`ID`) );

INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Jim', '1');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Tom', '1');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Harry', '2');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Dave', '2');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Tim', '3');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Trey', '4');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Jay', '4');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Steve', '4');
INSERT INTO `Players` (`Name`, `Team_ID`) VALUES ('Chris', '4');

我想要的是团队人数。

我想看下面的输出

Team_Size  Count
1          1
2          2
4          1

Answer 1

最简单的方法可能是：

select team_count, count(*) from
(select count(*) team_count from players group by team_id) sq
group by team_count

（尽管其中不包括没有球员的球队。）

SQLFiddle 在这里 。

Answer 2

首先，您需要团队规模：

select t.id as teamId, count(p.id) as teamSize
from 
    `Teams` as t
    left join `Players` as p on t.id = p.teamId
group by
    t.id;

请注意，这也将返回零个玩家的队伍。 如果您不想这样做，请使用inner join而不是left join 。

现在，将此查询用作最终查询的行源：

select teamSize, count(teamId)
from (
    select t.id as teamId, count(p.id) as teamSize
    from 
        `Teams` as t
        left join `Players` as p on t.id = p.teamId
    group by
        t.id) as a
group by teamSize;

希望这可以帮助

---

还有一件事。

如果您有大数据集，则此查询可能会挂起。 因此，最好创建一个临时表，对其进行索引，然后在临时表上运行查询：

drop table if exists temp_teamSize;
create temporary table temp_teamSize
    select t.id as teamId, count(p.id) as teamSize
    from 
        `Teams` as t
        left join `Players` as p on t.id = p.teamId
    group by
        t.id;
alter table temp_teamSize
    add unique index idx_teamId(teamId),
    add index idx_teamSize(teamSize);
select teamSize, count(teamId)
from temp_teamSize
group by teamSize;