在MySQL中獲取不同計數的計數

Question

我有兩個表，一個是團隊的，一個是玩家的，我想找出的是總人數表，換句話說，我想統計擁有2個成員的團隊總數，即所有團隊的總數有3個成員，等等

這是數據庫結構。

（側邊欄問題：我在這里是個新手：是否有更好的方法發布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');

我想要的是團隊人數。

我想看下面的輸出

Team_Size  Count
1          1
2          2
4          1

Answer 1

最簡單的方法可能是：

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

（盡管其中不包括沒有球員的球隊。）

SQLFiddle 在這里 。

Answer 2

首先，您需要團隊規模：

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;

請注意，這也將返回零個玩家的隊伍。 如果您不想這樣做，請使用inner join而不是left join 。

現在，將此查詢用作最終查詢的行源：

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;

希望這可以幫助

---

還有一件事。

如果您有大數據集，則此查詢可能會掛起。 因此，最好創建一個臨時表，對其進行索引，然后在臨時表上運行查詢：

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;