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A Select query will always return a result set object of zero or more records. There are several ways to then retrieve data from that result set, depending on the use case.

The most common case is to iterate over the result set with a foreach() loop.

$result = db_query("SELECT nid, title FROM {node}");
foreach ($result as $record) {
// Do something with each $record
$node = node_load($record->nid);

Depending on what the results are needed for, however, there are a number of other ways to retrieve the records.

To explicitly fetch the next record, use:

$record = $result->fetch();            // Use the default fetch mode.
$record = $result->fetchObject();  // Fetch as a stdClass object.
$record = $result->fetchAssoc();   // Fetch as an associative array.

If there is no next record, FALSE will be returned. fetch() should generally be avoided in favor of fetchObject() and fetchAssoc(), as the latter are more self-documenting. If you need to use some other PDO-supported fetch mode, then use fetch().

To fetch just a single field out of the the next record, use:

$record = $result->fetchField($column_index);

The default value of $column_index is 0, for the first field.

To count the number of rows returned from a DELETE, INSERT or UPDATE statement use:

$number_of_rows = $result->rowCount();

To count the number of rows returned from a SELECT statement use:

$number_of_rows = db_select('node')->countQuery()->execute()->fetchField();

To fetch all records at once into a single array, use one of the following:

// Retrieve all records into an indexed array of stdClass objects.
// Retrieve all records into an associative array keyed by the field in the result specified.
// Retrieve a 2-column result set as an associative array of field 0 => field 1.
// You can also specify which two fields to use by specifying the column numbers for each field
$result->fetchAllKeyed(0,2); // would be field 0 => field 2
$result->fetchAllKeyed(1,0); // would be field 1 => field 0
// If you need an array where keys and values contain the same field (e.g. for creating a 'checkboxes' form element), the following is a perfectly valid method:
$result->fetchAllKeyed(0,0); // would be field 0 => field 0, e.g. [article] => [article]
// Retrieve a 1-column result set as one single array.
// Column number can be specified otherwise defaults to first column

Note that fetchAll() and fetchAllAssoc() will by default fetch using whatever fetch mode was set on the query (numeric array, associative array, or object). That can be modified by passing in a new fetch mode constant. For fetchAll(), it is the first parameter. For fetchAllAssoc(), it is the second parameter. Examples:

// Get an array of arrays keyed on the field 'id'.
$result->fetchAllAssoc('id', PDO::FETCH_ASSOC);
// Get an array of arrays with both numeric and associative keys.

Because PHP supports chaining method calls on returned objects, it is very common to skip the $result variable entirely, like so:

// Get an array of node IDs.
$nids = db_query("SELECT nid FROM {node}")->fetchCol();
// Get an associative array of nids to titles.
$titles_by_nid = db_query("SELECT nid, title FROM {node}")->fetchAllKeyed();
// Get a single record out of the database.
$node = db_query("SELECT * FROM {node} WHERE nid = :nid", array(':nid' => $nid))->fetchObject();
// Get a single value out of the database.
$title = db_query("SELECT title FROM {node} WHERE nid = :nid", array(':nid' => $nid))->fetchField();