T-SQL subselect statement is returning all rows instead of limiting to 1 based on subselect - sql

Question

I am trying to return just the first row where the BLOCK_STOP_ORDER = 2. What is wrong with my SQL? Why isn't WHERE SCHEDULE.BLOCK_STOP_ORDER = (SELECT MIN(S1.BLOCK_STOP_ORDER....
working? When I run the subselect on its own it returns the value '2' - doesn't that mean it should then limit the query result to only the row(s) where BLOCK_STOP_ORDER = 2?
SELECT ROUTE.ROUTE_ABBR, SCHEDULE.ROUTE_DIRECTION_ID, SCHEDULE.PATTERN_ID, SCHEDULE.BLOCK_STOP_ORDER,
SCHEDULE.SCHEDULED_TIME, GEO_NODE.GEO_NODE_ABBR, TRIP.TRIP_SEQUENCE AS TPST
FROM SCHEDULE
INNER JOIN GEO_NODE ON SCHEDULE.GEO_NODE_ID = GEO_NODE.GEO_NODE_ID
INNER JOIN ROUTE ON SCHEDULE.ROUTE_ID = ROUTE.ROUTE_ID
INNER JOIN TRIP ON SCHEDULE.TRIP_ID = TRIP.TRIP_ID
WHERE (SCHEDULE.CALENDAR_ID = '120221024') AND ROUTE.ROUTE_ABBR = '001'
AND SCHEDULE.ROUTE_DIRECTION_ID = '2' AND SCHEDULE.PATTERN_ID = '270082'
AND TRIP.TRIP_SEQUENCE = '18600'
AND SCHEDULE.BLOCK_STOP_ORDER =
(SELECT MIN(S1.BLOCK_STOP_ORDER)
FROM SCHEDULE S1
WHERE SCHEDULE.CALENDAR_ID = S1.CALENDAR_ID
AND SCHEDULE.ROUTE_ID = S1.ROUTE_ID
AND SCHEDULE.ROUTE_DIRECTION_ID = S1.ROUTE_DIRECTION_ID
AND SCHEDULE.PATTERN_ID = S1.PATTERN_ID
AND SCHEDULE.SCHEDULED_TIME = S1.SCHEDULED_TIME
AND SCHEDULE.GEO_NODE_ID = S1.GEO_NODE_ID
AND SCHEDULE.BLOCK_STOP_ORDER = S1.BLOCK_STOP_ORDER
AND SCHEDULE.TRIP_ID = S1.TRIP_ID
)
GROUP BY ROUTE.ROUTE_ABBR, SCHEDULE.ROUTE_DIRECTION_ID,
SCHEDULE.PATTERN_ID, SCHEDULE.SCHEDULED_TIME,
GEO_NODE.GEO_NODE_ABBR, SCHEDULE.BLOCK_STOP_ORDER, TRIP.TRIP_SEQUENCE
ORDER BY ROUTE.ROUTE_ABBR, SCHEDULE.ROUTE_DIRECTION_ID, TRIP.TRIP_SEQUENCE
Results:
ROUTE_ABBR
ROUTE_DIRECTION_ID
PATTERN_ID
BLOCK_STOP_ORDER
SCHEDULED_TIME
GEO_NODE_ABBR
TPST
001
2
270082
2
18600
1251
18600
001
2
270082
3
18600
1346
18600
001
2
270082
5
18720
1123
18600
001
2
270082
6
18720
11372
18600
001
2
270082
4
18720
1570
18600
001
2
270082
8
18780
11373
18600

Answer 1

This is probably better solved with the row_number() windowing function:
SELECT *
FROM (
SELECT DISTINCT r.ROUTE_ABBR, s.ROUTE_DIRECTION_ID, s.PATTERN_ID, s.BLOCK_STOP_ORDER,
s.SCHEDULED_TIME, g.GEO_NODE_ABBR, t.TRIP_SEQUENCE AS TPST,
row_number() over (order by SCHEDULE.BLOCK_STOP_ORDER) rn
FROM SCHEDULE s
INNER JOIN GEO_NODE g ON s.GEO_NODE_ID = g.GEO_NODE_ID
INNER JOIN ROUTE r ON s.ROUTE_ID = r.ROUTE_ID
INNER JOIN TRIP t ON s.TRIP_ID = t.TRIP_ID
WHERE s.CALENDAR_ID = '120221024' AND r.ROUTE_ABBR = '001'
AND s.ROUTE_DIRECTION_ID = '2' AND s.PATTERN_ID = '270082'
AND t.TRIP_SEQUENCE = '18600'
) t1
WHERE rn=1
ORDER BY t1.ROUTE_ABBR, t1.ROUTE_DIRECTION_ID, t1.TRIP_SEQUENCE
The problem with the original is the name SCHEDULE. For the full version of the query, the subquery is matching the name in the nested select with the instance of the table from the outer select. This correlates the results of the inner table with the outer, so only the item from that row of the outer table is eligible.
When you run the inner query by itself, separate from the outer query, there is only the one instance of the table. In that situation the WHERE conditions are matching the table to itself — they are always true — and you just get the smallest value of all the rows: 2.
This is why you should ALWAYS give ALL the tables in your queries an alias, and ONLY reference them by that alias (as I did in my answer). Do this, and the MIN() version can work... but will still be slower and more code than using row_number().
Finally, the use of DISTINCT / GROUP BY with every SELECT column is usually an indicator you don't fully understand the JOIN relationships used in the query, and in at least one case the join conditions are not sufficiently selective. I'd hesitate to move a query like that to production, even if it seems to be working, though I confess most of us have done it at some point anyway.