添加外键约束时报错： SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails

1、添加外键约束时报错： SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails 。如图1

图1

PS E:\wwwroot\larabbs> php artisan migrate
Migrating: 2022_01_05_170327_add_references

   Illuminate\Database\QueryException

  SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`larabbs`.`#sql-13d0_5a1`, CONSTRAINT `topics_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE) (SQL: alter table `topics` add constraint `topics_user_id_foreign` foreign key (`user_id`) references `users` (`id`) on delete cascade)

  at E:\wwwroot\larabbs\vendor\laravel\framework\src\Illuminate\Database\Connection.php:703
    699▕         // If an exception occurs when attempting to run a query, we'll format the error
    700▕         // message to include the bindings with SQL, which will make this exception a
    701▕         // lot more helpful to the developer instead of just the database's errors.
    702▕         catch (Exception $e) {
  ➜ 703▕             throw new QueryException(
    704▕                 $query, $this->prepareBindings($bindings), $e
    705▕             );
    706▕         }
    707▕     }

  1   E:\wwwroot\larabbs\vendor\laravel\framework\src\Illuminate\Database\Connection.php:492
      PDOException::("SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`larabbs`.`#sql-13d0_5a1`, CONSTRAINT `topics_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE)")

  2   E:\wwwroot\larabbs\vendor\laravel\framework\src\Illuminate\Database\Connection.php:492
      PDOStatement::execute()
PS E:\wwwroot\larabbs>

2、分析原因，用户ID等于 9 的记录在 topics 中存在，但是在 users 中已经不存在所导致。如图2

图2

3、最终决定删除掉 topics 中用户ID等于 9 的记录。再次执行命令，不再报错，外键约束 topics_user_id_foreign 已经添加成功。如图3

图3