Drupal 7 Module Development

Database Access
Dynamic queries
Although most SELECT queries are static, at times we will need a more flexible
query. That could be because the query itself may change depending on incoming
user data, because we want to allow other modules to modify our query before
it is executed, or we want to take advantage of some database feature that is
implemented differently on different databases. For these cases, Drupal provides
a mechanism for building dynamic queries using a robust query builder.
To start, we create a new query object with db_select():
$query = db_select('node', 'n');
The first parameter is the name of the base table of the query and the second is the
alias we want to use for it. We then call additional methods on the $query object in
order to build up the query logic we want to create dynamically. For example:
$query = db_select('node', 'n');
$query->fields('n', array('nid, title'));
$u_alias = $query->innerJoin('users' ,'u', '%alias.uid = n.uid');
$query->addField($u_alias, 'name', 'username');
$query->condition("{$u_alias}.name", 'Bob');
$query->condition('n.created', REQUEST_TIME - 604800, '>=');
$query->orderBy('n.created', 'DESC');
$query->range(0, 5);
$query->addTag('node_access');
$result = $query->execute();
The fields() method tells the query to select the fields in the second parameter
from the table in the first parameter. In this case, as of line 2, our query would
effectively read:
SELECT n.nid AS nid, n.title AS title FROM {node} n
Note that the curly braces are added for us automatically. Also note that aliases are
created for every field. If we want to specify an alternate alias, we need to use the
addField() method for that one field. We'll see more on that shortly. We can also
join against other tables using the innerJoin(), leftJoin(), and rightJoin()
methods. There is also join(), which is an alias of innerJoin(). The join()
methods take the table to join against, its alias, and the join conditions as parameters
in the form of an SQL fragment. Note that in this case we are using the string %alias
in the join clause. That's because while we are joining against the users table and
asking for an alias of "u", we may end up with a different alias if that alias already
exists in this query. Although we're quite sure that's not the case here, it could be
the case in query_alter() hooks, so it's a good habit to get into.
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