laravel关联查询

1.创建表：

-- 创建学生表
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 NOT NULL DEFAULT '' COMMENT '姓名',
  `age` tinyint(3) NOT NULL DEFAULT '0' COMMENT '年龄',
  `sex` tinyint(3) NOT NULL DEFAULT '0' COMMENT '性别，0：未知，1：男，2：女',
  `created_at` int(11) NOT NULL DEFAULT '0' COMMENT '创建时间',
  `updated_at` int(11) NOT NULL DEFAULT '0' COMMENT '修改时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='学生表';

INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (3, '小王', 18, 2, 1641469262, 1641469262);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (10, '小张', 18, 1, 1641469804, 1641529381);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (11, '小李', 19, 1, 1641469904, 1641529381);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (14, '小朱', 19, 1, 1641525262, 1641529381);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (15, '小杜', 18, 1, 1641526322, 1641529381);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (17, '小梁', 17, 1, 1641526483, 1641529381);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (18, '小吴', 18, 1, 1641527008, 1641529381);
INSERT INTO `student`(`id`, `name`, `age`, `sex`, `created_at`, `updated_at`) VALUES (19, '小周', 17, 1, 1641527008, 1641529381);

-- 创建学生信息表
CREATE TABLE `student_info` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) NOT NULL DEFAULT '0' COMMENT '学生id',
  `email` varchar(60) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '邮箱',
  `address` varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '地址',
  `created_at` int(11) NOT NULL DEFAULT '0' COMMENT '创建时间',
  `updated_at` int(11) NOT NULL DEFAULT '0' COMMENT '修改时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='学生信息表';

INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (1, 3, 'test3@test.com', '山东省济南市历下区', 1642404313, 1642404313);
INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (2, 10, 'test10@test.com', '天津市南开区', 1642404313, 1642404313);
INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (3, 11, 'test11@test.com', '河北省石家庄市裕华区', 1642404313, 1642404313);
INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (4, 14, 'test14@test.com', '河北省廊坊市广阳区', 1642404313, 1642404313);
INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (5, 15, 'test15@test.com', '山东省青岛市南市区', 1642404313, 1642404313);
INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (6, 17, 'test17@test.com', '河南省郑州市中原区', 1642404313, 1642404313);
INSERT INTO `student_info`(`id`, `student_id`, `email`, `address`, `created_at`, `updated_at`) VALUES (7, 18, 'test18@test.com', '河南省南阳市卧龙区', 1642404313, 1642404313);

-- 创建考试结果表
CREATE TABLE `exam` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `exam_name` varchar(60) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '科目',
  `fraction` decimal(5,2) NOT NULL DEFAULT '0.00' COMMENT '考试成绩',
  `student_id` int(11) NOT NULL DEFAULT '0' COMMENT '学生id',
  `created_at` int(11) NOT NULL DEFAULT '0' COMMENT '创建时间',
  `updated_at` int(11) NOT NULL DEFAULT '0' COMMENT '修改时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='考试结果表';

INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (1, '数学', 67.00, 3, 1642402349, 1642402349);
INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (2, '数学', 100.00, 10, 1642402349, 1642402349);
INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (3, '数学', 98.00, 11, 1642402349, 1642402349);
INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (4, '数学', 63.00, 14, 1642402349, 1642402349);
INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (5, '数学', 96.00, 15, 1642402349, 1642402349);
INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (6, '数学', 78.00, 17, 1642402349, 1642402349);
INSERT INTO `exam`(`id`, `exam_name`, `fraction`, `student_id`, `created_at`, `updated_at`) VALUES (7, '数学', 77.00, 18, 1642402349, 1642402349);

-- 创建创建角色表
CREATE TABLE `roles` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `role_name` varchar(60) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '角色名称',
  `created_at` int(11) NOT NULL DEFAULT '0' COMMENT '创建时间',
  `updated_at` int(11) NOT NULL DEFAULT '0' COMMENT '修改时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='角色表';

INSERT INTO `roles`(`id`, `role_name`, `created_at`, `updated_at`) VALUES (1, '班长', 1642411155, 1642411155);
INSERT INTO `roles`(`id`, `role_name`, `created_at`, `updated_at`) VALUES (2, '副班长', 1642411155, 1642411155);
INSERT INTO `roles`(`id`, `role_name`, `created_at`, `updated_at`) VALUES (3, '班委', 1642411155, 1642411155);
INSERT INTO `roles`(`id`, `role_name`, `created_at`, `updated_at`) VALUES (4, '课代表', 1642411155, 1642411155);

-- 创建学生角色表
CREATE TABLE `student_role` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `role_id` int(11) NOT NULL DEFAULT '0' COMMENT '角色id',
  `student_id` int(11) NOT NULL DEFAULT '0' COMMENT '学生id',
  `created_at` int(11) NOT NULL DEFAULT '0' COMMENT '创建时间',
  `updated_at` int(11) NOT NULL DEFAULT '0' COMMENT '修改时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='学生角色表';

INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (1, 1, 3, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (2, 2, 10, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (3, 3, 11, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (4, 4, 14, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (5, 4, 15, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (6, 4, 17, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (7, 4, 18, 1642413366, 1642413366);
INSERT INTO `student_role`(`id`, `role_id`, `student_id`, `created_at`, `updated_at`) VALUES (8, 4, 3, 1642413366, 1642413366);

2.一对一关联查询：

//student表一对一正向关联student_info表：在student表model中添加关联方法：
public function studentInfo() {
    return $this->hasOne(StudentInfo::class, 'student_id', 'id');
}
//查询数据：
$list = Student::find(20)->studentInfo;

//student_info表一对一反向关联student表：在student_info表model中添加关联方法：
public function student() {
    return $this->belongsTo(Student::class, 'student_id', 'id');
}
//查询数据：
$list = StudentInfo::find(3)->student;

3.一对多关联查询：

//student表一对多正向关联exam表：在student表model中添加关联方法：
public function exam() {
    return $this->hasMany(Exam::class, 'student_id', 'id');
}
//查询数据：
$list = Student::find(3)->exam;    //一对多，不带查询条件
$list = Student::find(3)->exam()->where('fraction', '>=', 80)->get(); //一对多，带查询条件

//student表一对多反向关联exam表：在exam表model中添加关联方法：
public function student() {
    return $this->belongsTo(Student::class, 'student_id', 'id');
}
//查询数据：
$list = Exam::find(3)->student;

4.多对多关联查询：

//student表多对多关联roles表（通过student_role中间表进行关联，student_id和role_id字段为student_role表中的字段）：在student表model中添加关联方法：
public function roles() {
    return $this->belongsToMany(Roles::class, 'student_role', 'student_id', 'role_id');
}

//查询数据：
$list = Student::find(3)->roles;

//可以将SQL语句打印出来查看下是否正确
$sql = Student::find(3)->roles()->toSql();

//增加查询条件：
$list = Student::find(3)->roles()->where(function($query) {
    $query->where('role_name', '课代表');
})->get();

//has：判断student表与roles表是否存在关联数据。参数一为model名，当前查询信息为student与role表存在关联数据小于1的，若只写roles参数，则默认为大于等于1。
$list = Student::has('roles', '>', 1)->get();

//whereHas：使用闭包组合查询条件
$list = Student::whereHas('roles', function ($query) {
    $query->where('role_name', '班长');
})->get();

//doesntHave，has的反向操作，判断student表与roles表不存在关联的数据。
$list = Student::doesntHave('roles')->get();

//withCount：统计student表每条数据与roles表关联数据数量
$list = Student::withCount('roles')->get();
//doesntHave使用时设置别名，增加查询条件
$list = Student::withCount(['roles as role_count'=>function($query) {
    $query->where('roles.id', 1);
}])->get();

参考文档：
https://learnku.com/docs/laravel/8.x/eloquent-relationships/9407

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