Drupal的数据库层，包含了有关占位符的一个附加特性。对于一个占位符，如果传递给它的值是一个数组，它就会自动地将占位符扩展为逗号分隔的占位符列表，分别对应于数组中的元素。这意味着开发者不需要考虑他们需要多少个占位符。
我们通过实例来学习一下这一特性：
<?php
// This code:
db_query("SELECT * FROM {node} WHERE nid IN (:nids)", array(':nids' => array(13, 42, 144));
// Will get turned into this prepared statement equivalent automatically:
db_query("SELECT * FROM {node} WHERE nid IN (:nids_1, :nids_2, :nids_3)", array(
':nids_1' => 13,
':nids_2' => 42,
':nids_3' => 144,
));
// Which is equivalent to the following literal query:
db_query("SELECT * FROM {node} WHERE nid IN (13, 42, 144)");
?>