Impossible d'effectuer la requête

Voici la requête :
SELECT inf_clicks FROM an_values WHERE tb_id= and enr_id= and inf_id=

Voici le message d'erreur renvoyé par la base de données :
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 'and enr_id= and inf_id=' at line 1
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /mnt/sites/havre-achat.com/web/click.php on line 33