custom_sql_migrate_source_plugin

Prerequisites:
Existing database entered in the settings.php file.
In this example:

$databases['mg_legacy']['default'] = array (
  'database' => 'mg_legacy',
  'username' => 'xxx',
  'password' => 'xxx',
  'host' => 'localhost',
  'port' => '3306',
  'driver' => 'mysql',
  'prefix' => '',
  'collation' => 'utf8mb4_general_ci',
);

Installation:
Enable module

Description:
With this CustomSQLQuery plugin you can enter a custom SQL string in your migration config file and it will pull the data directly from any database.

It returns all the fields queried in the SQL string in the row object. So any field you select in the SQL query can be accessed by any process plugin or as a source for any field.

IMPORTANT: Any changes to the sql requires you to re-install your custom migration module for the settings to take affect. You can do so with

drush pmu [module_name] -y && drush en [module_name] -y

Please see config below as an example:

id: resources
label: Resources
migration_group: mmg8_legacy_migrations
migration_dependencies:
  required:
    - mmg8_legacy_migrations_article_type
source:
  plugin: custom_sql_query
  key: mg_legacy
  keys:
    - id
  sql_query: 'SELECT resources.id,
                     CONCAT(''/'', resources.slug) as slug,
                     resources.title,
                     resources.description,
                     resources.file,
                     resources.published,
                     resources.category_id,
                     resources.created_at,
                     resources.updated_at,
                     resources.locked,
                     resources.subtitle,
                     resources.archive
                FROM resources resources'
destination:
  plugin: entity:node
  default_bundle: article
process:
  title: title
  path/pathauto:
    plugin: default_value
    default_value: 0 # Disable pathauto.
  path/alias: slug