Sub_select problem

Good day, everyone, I have 2 tables and 1 datafile as following: vehicle_tb ----------- v_id int4 primary key display_id int4 year text make text model text ... vehicle_borrow_log -------------------- employee_id int4 (foreign key to emp_tb) v_id int4 (foreign key to vehicle_tb) borrow_time timestamp return_time timestamp ... datafile.gnu ( a text file) --------------------------- # Date Display_id 2000-XX-XX 1 2000-xx-xx 1 2000-XX-XX 2 2000-xx-xx 2 2000-XX-XX 1 2000-xx-xx 1 ... What I want to do is to fetch the display_id in datafile using PHP (by the way, this part is done). And then use that display_id to get the borrow_time and return time in vehicle_borrow_log. So, I used this sub-select SQL, but it doesn't seem to be working. SELECT v_id, borrow_time, return_time FROM vehicle_borrow_log vbl WHERE (SELECT equip_id FROM vehicle_tb vtb WHERE vtb.v_id=vbl.v_id AND display_id=$id_from_datafile_let_say_it_is_2); Could anyone please help? Thanks! - Bernie