Handle database schemas on Drupal 8

Get Database Schema

$schema = \Drupal\Core\Database\Database::getConnection()->schema();

Create a table.

$spec = array(
'description' => 'New table description',
'fields' => array(
  'id' => array(
    'type' => 'serial',
    'not null' => TRUE,
  ),
  'name' => array(
    'type' => 'varchar',
    'length' => 64,
    'not null' => TRUE,
    'default' => '',
  ),
  'value' => array(
    'type' => 'varchar',
    'length' => 64,
    'not null' => TRUE,
    'default' => '',
  ),
),
'primary key' => array('id'),
);
$schema->createTable('the_table_name', $spec);
 
 

Drop / Delete a field.

$schema = Database::getConnection()->schema();
$schema->dropField('the_table_name', 'field_to_drop');

Add a field to a table.

$spec = array(
    'type' => 'varchar',
    'length' => '255',
    'not null' => TRUE,
    'default' => '0',
);
$schema->addField('the_table_name', 'field_to_add', $spec);

Examples

Delete a table field from hook_update_N()

/**
 * Update 8001, Delete Field 'field_to_delete' from table 'the_test_table'.
 */
function mymodule_update_8001(&$sandbox) {
    $schema = \Drupal\Core\Database\Database::getConnection()->schema();
    $schema->dropField('the_test_table', 'field_to_delete');

    return ('The field field_to_delete has been dropped from the_test_table');
}

More : https://api.drupal.org/api/drupal/includes!database!schema.inc/group/schemaapi/7.x