php - Laravel 中可空列作为外键

Question

我的数据库中有客户表。一个客户与一个行业相关联。由于要求，创建客户时，industry_id 为空。稍后将更新并添加正确的行业 ID。

现在，当我想将此列添加为外键时，它会显示以下错误。

SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table customers add constraint customers_industry_id_foreign foreign key (industry_id) references industries
(id))

我在客户迁移中有以下代码。

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateCustomersTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('customers', function (Blueprint $table) {
            $table->bigIncrements('id')->comment="Customer Identifier";
            $table->bigInteger('customer_category_id')->unsigned()->comment="Customer Category Identifier";
            $table->bigInteger('industry_id')->unsigned()->nullable()->comment="Industry Identifier";
            $table->string('email')->unique()->comment="Customer Email";
            $table->timestamps();

            $table->foreign('industry_id')->references('id')->on('industries');
            $table->foreign('customer_category_id')->references('id')->on('customer_categories');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('customers');
    }
}

这是产业迁移。

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateIndustriesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('industries', function (Blueprint $table) {
            $table->bigIncrements('id')->comment="Industry Indetifier";
            $table->string('name')->comment="Industry Name";
            $table->boolean('status')->default(true)->comment="Active or Inactive";
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('industries');
    }
}

难道我想要的东西就无法实现吗？或者这根本不合逻辑？如果我能够添加外键，那么我可以利用使用外键的各种优势。

Answer 1

将行业表中的 id(primary_key) 更改为唯一键，然后尝试