selectCheckBox and type check fields compared

There is a subtle difference between select fields with the render type selectCheckBox and fields of the type check:

Select values from a checkbox list

The select checkbox stores the values as comma separated values.

        EXT:my_extension/Configuration/TCA/Overrides/332-tx_myextension_speaker-topics.php
    

<?php

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

defined('TYPO3') or die();

ExtensionManagementUtility::addTCAcolumns('tx_myextension_speaker', [
  'topics' => [
    'label' => 'my_extension.db:speaker.topics',
    'config' => [
      'type' => 'select',
      'renderType' => 'selectCheckBox',
      'items' => [
        [
          'label' => 'my_extension.db:speaker.topics.tca',
          'value' => 'tca',
          'group' => 'backend',
          'icon' => 'content-database',
          'description' => 'my_extension.db:speaker.topics.tca.description',
        ],
        ['label' => 'my_extension.db:speaker.topics.extbase', 'value' => 'extbase', 'group' => 'backend'],
        ['label' => 'my_extension.db:speaker.topics.fluid', 'value' => 'fluid', 'group' => 'frontend'],
        ['label' => 'my_extension.db:speaker.topics.typoscript', 'value' => 'typoscript', 'group' => 'frontend'],
        ['label' => 'my_extension.db:speaker.topics.accessibility', 'value' => 'accessibility', 'group' => 'frontend'],
      ],
      'itemGroups' => [
        'backend' => 'my_extension.db:speaker.topics.group.backend',
        'frontend' => 'my_extension.db:speaker.topics.group.frontend',
      ],
      'appearance' => [
        'expandAll' => true,
      ],
      'behaviour' => [
        'allowLanguageSynchronization' => true,
      ],
    ],
  ],
]);
ExtensionManagementUtility::addToAllTCAtypes(
  'tx_myextension_speaker',
  'topics',
  '',
  'after:short_bio',
);

The field in the database is of type text or varchar.

CREATE TABLE tx_myextension_speaker (
  topics text
);

Select values from a checkbox list

On the contrary the type check saves multiple values as bits. Therefore if the first value is chosen it stores 1 (binary 00000001), if only the second is chosen it stores 2 (binary 00000010) and if both are chosen 3 (binary 00000011).

        EXT:my_extension/Configuration/TCA/Overrides/136-tx_myextension_conference-amenities.php
    

<?php

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

defined('TYPO3') or die();

ExtensionManagementUtility::addTCAcolumns('tx_myextension_conference', [
  'amenities' => [
    'label' => 'my_extension.db:conference.amenities',
    'config' => [
      'type' => 'check',
      'items' => [
        ['label' => 'my_extension.db:conference.amenities.catering'],
        ['label' => 'my_extension.db:conference.amenities.childcare'],
        ['label' => 'my_extension.db:conference.amenities.live_stream'],
        ['label' => 'my_extension.db:conference.amenities.wheelchair_access'],
      ],
      'cols' => 3,
    ],
  ],
]);
ExtensionManagementUtility::addToAllTCAtypes(
  'tx_myextension_conference',
  'amenities',
  '',
  'after:ticketing_secret',
);

The field in the database is of type int.

CREATE TABLE tx_myextension_conference (
  amenities int(11) DEFAULT '0' NOT NULL
);