In Drupal 8, the usage of database operations such as select, update and delete are slightly changed from Drupal 7. The db_query() is deprecated in Drupal 8. To fetch a field we can use :
$query = \Drupal::database()->query('SELECT myfield from my_table where
field1 = :value1 AND field2 = :value2', array(
':value1' => $condition1,
':value2' => $condition2,
)
);
$data = $query->fetchField();
Now we can checkout the select, update, and delete operations in Drupal 8. For a field selection use,