Drupal 7 db_select 嵌套选择查询

使用Drupal 7的db_select函数进行数据库查询时，有时候需要进行嵌套选择查询。嵌套选择查询在一些复杂的数据关联情况下非常有用，可以轻松地获取所需的数据。下面将介绍如何使用嵌套选择查询，并提供一个实际案例代码。

$query = db_select('table1', 't1');
$query->join('table2', 't2', 't1.id = t2.t1_id');
$query->fields('t1', array('field1', 'field2'));
$subquery = db_select('table3', 't3');
$subquery->join('table4', 't4', 't3.id = t4.t3_id');
$subquery->fields('t3', array('field3', 'field4'));
$query->condition('t1.id', $subquery, 'IN');
$results = $query->execute()->fetchAll();

$query = db_select('node', 'n');
$query->join('field_data_comment_body', 'c', 'n.nid = c.entity_id');
$query->fields('n', array('title', 'created'))
  ->fields('c', array('comment_body_value'));

$subquery = db_select('field_data_comment_body', 'c2');
$subquery->fields('c2', array('entity_id'))
  ->condition('comment_body_value', '%关键字%', 'LIKE');

$query->condition('n.nid', $subquery, 'IN');
$results = $query->execute()->fetchAll();

$query = db_select('node', 'n');
$query->join('field_data_comment_body', 'c', 'n.nid = c.entity_id');
$query->fields('n', array('title', 'created'))
  ->fields('c', array('comment_body_value'));
$subquery = db_select('field_data_comment_body', 'c2');
$subquery->fields('c2', array('entity_id'))
  ->condition('comment_body_value', '%关键字%', 'LIKE');
$query->condition('n.nid', $subquery, 'IN');
$results = $query->execute()->fetchAll();