我正在使用标志模块，让用户将他们的问题标记为已解决，类似于其在本网站上的工作方式。因此，我希望用户过滤帖子以仅显示未解决的问题。此外，某些帖子可能根本不是问题。因此，它们不能具有“已解决”标志，但是当仅按未解决的问题进行过滤时，它们也不应出现在结果中。因此，我需要将节点表与其他两个表连接起来：flag_content和field_data_field_question（后面的表说明了帖子是否是一个问题）。
这是我正在尝试的当前代码：
$query->join('flag_content', 'f', 'f.content_id = n.nid AND f.content_type = "node" AND f.fid = 5');
$query->join('field_data_field_question', 'q', 'q.entity_id = n.nid AND q.field_question_value = 1');
$query->condition('f.fid', 'NULL', 'IS');
$query->condition('q.field_question_value', 'NULL', 'IS NOT');但是，这导致以下错误：
PDOException: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''NULL') AND (q.field_question_value IS NOT 'NULL')
LIMIT 21 OFFSET 0' at line 4: SELECT n.nid AS nid
FROM
{node} n
INNER JOIN {field_data_field_category} t ON t.entity_id = n.nid
INNER JOIN {flag_content} f ON f.content_id = n.nid AND f.content_type = "node" AND f.fid = 5
INNER JOIN {field_data_field_question} q ON q.entity_id = n.nid AND q.field_question_value = 1
WHERE (t.field_category_tid = :db_condition_placeholder_0) AND (f.fid IS :db_condition_placeholder_1) AND (q.field_question_value IS NOT :db_condition_placeholder_2)
LIMIT 21 OFFSET 0; Array
( [:db_condition_placeholder_0] => 464 [:db_condition_placeholder_1] => NULL [:db_condition_placeholder_2] => NULL
)
in queryExecuteRender_recentActivity() (line 57 of someFile.php).这不是将NULL值传递给查询的正确方法吗？