Mysql solves the database N+1 query problem

<resultMap id="BaseResultMap" type="testmaven.entity.User">
  <id column="id" jdbcType="INTEGER" property="id" />
  <result column="name" jdbcType="VARCHAR" property="name" />
  <result column="age" jdbcType="INTEGER" property="age" />
  <result column="dept_id" jdbcType="INTEGER" property="deptId" />
  <association property="dept" column="dept_id" fetchType="eager" select="testmaven.mapper.DeptMapper.selectByPrimaryKey" ></association>
 </resultMap>

[
  { "id":1, "name":"test", "department_id":1, "department":{ "id":1, "name":"Test Department"
    }
  }
]

$users = $db->query('SELECT * FROM `user`'); foreach($users as &$user) {
  $users['department'] = $db->query('SELECT * FROM `department` WHERE `id` = '.$user['department_id']);
}

$users = $db->query('SELECT * FROM `user` INNER JOIN `department` ON `department`.`id` = `user`.`department_id`'); // Manual processing returns the result as required structure

$users = $db->query('SELECT * FROM `user`');
$departmentIds = [ ]; foreach($users as $user) { if(!in_array($user['department_id'], $departmentIds)) {
    $departmentIds[] = $user['department_id'];
  }
}
$departments = $db->query('SELECT * FROM `department` WHERE id in ('.join(',',$department_id).')');
$map = []; // [department ID => department item]foreach($departments as $department) {
  $map[$department['id']] = $department;
}foreach($users as $user) {
  $user['department'] = $map[$user['department_id']] ?? null;
 }