$wpdb->get_results("SELECT ...", ARRAY_A);
ARRAY_A is a "output_type" argument. It can be one of four pre-defined constants (defaults to OBJECT):
OBJECT - result will be output as a numerically indexed array of row objects.
OBJECT_K - result will be output as an associative array of row objects, using first columns values as keys (duplicates will be discarded).
ARRAY_A - result will be output as an numerically indexed array of associative arrays, using column names as keys.
ARRAY_N - result will be output as a numerically indexed array of numerically indexed arrays.