+ of these criteria is determined as follows:
+ * if the dictionnary has the '-AND' or the '-OR' key, this is chosen
+ * otherwise, the join_with argument, as provided to the sql method below,
+ is expected to hold the 'AND' or 'OR' string
+ this argument defaults to 'AND' and in most of the code, this default applies
+ as the join_with argument is left unspecified
+