drupal7使用数据库api db_query需要注意的地方

写自定义module时候需要从数据库检索数据，用到了数据库的api，用了下面的sql：

$record =  db_query("SELECT 'sampledate', 'time' FROM {table_name} WHERE uid = :uid", array(':uid' => $uid))->fetchObject();

结果var_dump($record);得到的结果为

object(stdClass)#221 (2) { ["sampledate"]=> string(10) "sampledate" ["heartrate"]=> string(9) "heartrate" } 

思来想去，“百撕不得骑姐”，无奈只好google之，在官网问答里面的到了启发：

You must use single quotes

This is new in Drupal 7. Although MySQL doesn't care about single or double quotes, other SQL engines do.

Works:

SELECT delta FROM {block} WHERE status = 0 AND module = 'tracker'

Does not work:

SELECT delta FROM {block} WHERE status = 0 AND module = "tracker"

咋一看之下，两个SQL并无差异。只是‘ 与 “ 的分别。原来drupal7里面SQL要使用single quotes,于是把SQL该了一下，去掉 ‘ （单引号），得到的结果就正常了：

$record =  db_query("SELECT sampledate, time FROM {table_name} WHERE uid = :uid", array(':uid' => $uid))->fetchObject();