SELECT * FROM table1 LEFT JOIN table2 ON table2 .folder_id = table1.id
WHERE members_folders.mem_id=’1100′ AND ISNULL(table2.id)
-get records from table1 that doesn’t have a record on table2
ISNULL() – Test whether the argument is NULL
source: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_isnull