Mysql 尝试求和多次和

我试着这样求多个和：

    SELECT
    `users`.*,
    @attackPower := FLOOR(SUM(IF( `weapons`.`weaponeType` = 'atk',`weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `attackPower` ,
    @defencePower := FLOOR(SUM(IF( `weapons`.`weaponeType` = 'def', `weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `defencePower`,
    @spyPower := FLOOR(SUM(IF( `weapons`.`weaponeType` = 'spy', `weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `spyPower`,
    @uniPower := FLOOR(SUM(IF( `weapons`.`weaponeType` = 'uni', `weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `uniPower`,
    (@attackPower + @defencePower + @spyPower + @uniPower) as `power`

    FROM `users`
    LEFT JOIN `weapons_has_users` ON `users`.`userId` = `weapons_has_users`.`userId`
    LEFT JOIN `weapons` ON `weapons`.`weaponId` = `weapons_has_users`.`weaponId`

但是

power

返回NULL 我能做些什么来解决这个问题？

MySQL文档特别警告不要在同一语句中赋值和使用变量

此外，如果要在查询中使用聚合函数（并且希望返回多行），则需要一个子查询

因为您需要使用聚合，所以也可以使用子查询：

select u.*,
    (attackPower + defencePower + spyPower + uniPower) as `power`
from (SELECT `users`.*,
             FLOOR(SUM(IF( `weapons`.`weaponeType` = 'atk',`weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `attackPower` ,
             FLOOR(SUM(IF( `weapons`.`weaponeType` = 'def', `weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `defencePower`,
             FLOOR(SUM(IF( `weapons`.`weaponeType` = 'spy', `weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `spyPower`,
             FLOOR(SUM(IF( `weapons`.`weaponeType` = 'uni', `weapons`.`weaponPower` * `weapons_has_users`.`count` , 0 ) )) AS `uniPower`,

      FROM `users` LEFT JOIN
           `weapons_has_users`
           ON `users`.`userId` = `weapons_has_users`.`userId` LEFT JOIN
           `weapons`
           ON `weapons`.`weaponId` = `weapons_has_users`.`weaponId`
      GROUP BY users.id
    ) u;