Unknown column error accessing outer query column inside subquery - mysql

Question

I don't know how to explain my problem better than just showing the code and the error.
The following query is a minimal example for the problem I have encountered
UPDATE stops AS stops1 SET real_stop_id = (
SELECT MIN(served_by2.stop_id)
FROM served_by AS served_by1
INNER JOIN served_by AS served_by2
USING(route_short_name)
WHERE served_by1.stop_id = stops.stop_id
);
It returns the following error.
ERROR 1054 (42S22): Unknown column 'stops.stop_id' in 'where clause'
I'm using the timetable data for my city's tram network in the Google Transit Feed Specification format
https://developers.google.com/transit/gtfs/reference
In addition to this I have created and populated a table served_by.
mysql> describe served_by;
+------------------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+---------+------+-----+---------+-------+
| stop_id | int(11) | YES | | NULL | |
| route_short_name | text | YES | | NULL | |
+------------------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> select * from served_by limit 5;
+---------+------------------+
| stop_id | route_short_name |
+---------+------------------+
| 378 | 19 |
| 378 | 19 |
| 378 | 19 |
| 378 | 19 |
| 398 | 5 |
+---------+------------------+
5 rows in set (0.06 sec)
For completeness, here is the stops table, which follows the GTFS format except for the column real_stop_id (which has no data), that I am currently trying to populate.
mysql> describe stops;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| stop_id | int(11) | YES | MUL | NULL | |
| real_stop_id | int(11) | YES | | NULL | |
| stop_no | varchar(30) | YES | MUL | NULL | |
| stop_name | varchar(150) | YES | MUL | NULL | |
| stop_lat | double | YES | | NULL | |
| stop_lon | double | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)
mysql> select * from stops limit 5;
+----+---------+--------------+---------+--------------------------------------------+-------------------+------------------+
| id | stop_id | real_stop_id | stop_no | stop_name | stop_lat | stop_lon |
+----+---------+--------------+---------+--------------------------------------------+-------------------+------------------+
| 1 | 10311 | 947 | 45 | 45-Glenferrie Rd/Wattletree Rd (Malvern) | -37.862296736384 | 145.028194441473 |
| 2 | 10371 | 946 | 44 | 44-Duncraig Ave/Wattletree Rd (Armadale) | -37.8618932396197 | 145.025090664641 |
| 3 | 1083 | 1083 | 42 | 42-Clyde St/Raleigh Rd (Maribyrnong) | -37.7696986370071 | 144.898841257316 |
| 4 | 11285 | 940 | 43 | 43-Egerton Rd/Wattletree Rd (Armadale) | -37.8615917660895 | 145.02270030676 |
| 5 | 1185 | 1185 | 50 | 50-Vincent St/Wattletree Rd (Malvern East) | -37.8643850684538 | 145.04371198053 |
+----+---------+--------------+---------+--------------------------------------------+-------------------+------------------+
5 rows in set (0.06 sec)

Answer 1

You have renamed it to stops1. You need to use the alias instead of the original table name:
UPDATE stops s
SET real_stop_id = (SELECT MIN(sb2.stop_id)
FROM served_by sb1 JOIN
served_by sb2
USING (route_short_name)
WHERE sb1.stop_id = s.stop_id
);
I find that table abbreviations are easier to write and read.