Add custom mapping tables for a migration process that merges terms - these tables will be used to import data from a csv file

my_migration.install.php

<?php

/**
 * Add legacy and new term name mapping for Section and Subject vocabularies
 */
function my_migration_update_8001() {
  $section_schema  = [
    'description' => 'Legacy section term name to new section term name',
    'fields' => [
      'legacy_id' => [
        'type' => 'serial',
        'not null' => TRUE,
        'description' => 'Primary Key: Unique person ID.',
      ],
      'old_name' => [
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
        'description' => 'Name of the person.',
      ],
      'new_name' => [
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
        'description' => 'Name of the person.',
      ],
    ],
    'primary key' => ['legacy_id'],
    'indexes' => [
      'old_name' => ['old_name'],
      'new_name' => ['new_name'],
    ],
  ];
  db_create_table('my_migration_section', $section_schema);
  
  $subject_schema  = [
    'description' => 'Legacy section term name to new section term name',
    'fields' => [
      'legacy_id' => [
        'type' => 'serial',
        'not null' => TRUE,
        'description' => 'Primary Key: Unique person ID.',
      ],
      'old_name' => [
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
        'description' => 'Name of the person.',
      ],
      'new_name' => [
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
        'description' => 'Name of the person.',
      ],
    ],
    'primary key' => ['legacy_id'],
    'indexes' => [
      'old_name' => ['old_name'],
      'new_name' => ['new_name'],
    ]
  ];
  db_create_table('my_migration_subject', $subject_schema);
}