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