JOIN results in too many rows - sql

Question

I would be super happy if I get help for this problem. Thank you in advance.
Table #1: station_temporar_con_station has 5984 rows, and 7 columns as seen in the screenshot:
ID_stations, latitude, longitude, connection_coord_city_type_coordinates_text, type_of_stations, ID_city
SQL_station_temporar_con_station
Table #2: air_quality_temporar has 11946 rows and 13 columns as seen in this screenshot:
table air_quality_temporar
Now I should have a table with all the 11946 rows from air_quality_temporar supplemented with the column connection_coord_city_type from station_temporar_con_station.
What I've tried so far:
Solution #1:
SELECT
ID_measurement, ID_stations,
station_temporar_con_station.latitude,
station_temporar_con_station.longitude,
station_temporar_con_station.connection_coord_city_type,
station_temporar_con_station.coordinates_text,
type_of_stations, ID_city
FROM
station_temporar_con_station
JOIN
air_quality_temporar ON station_temporar_con_station.coordinates_text = air_quality_temporar.coordinates_text;
But this JOIN results in 14'377 rows instead of 11'946 rows.
Solution #2:
SELECT
reference, pm25, PM10, latitude, longitude,
(SELECT connection_coord_city_type
FROM station_temporar_con_station),
conc_pm25, conc_pm10, year, pm10_type, pm25_type, date_compiled
FROM
air_quality_temporar;
But only the first value from connection_cord_city_type is filled in, because the DB does not know what it should assign where.
Does anyone have any input or a solution?

Answer 1

You should try to avoid duplicate connections and join only unique data. I added two latitude and longitude fields to the join below.
I also used the left join and the air_quality_temporar table in put left table To recover 11946 rows.
SELECT ID_measurement, ID_stations,
station_temporar_con_station.latitude,
station_temporar_con_station.longitude,
station_temporar_con_station.connection_coord_city_type,
station_temporar_con_station.coordinates_text, type_of_stations, ID_city
FROM air_quality_temporar
LEFT JOIN station_temporar_con_station
ON air_quality_temporar.coordinates_text = station_temporar_con_station.coordinates_text
AND air_quality_temporar.latitude = station_temporar_con_station.latitude AND
air_quality_temporar.longitude = station_temporar_con_station.longitude

Answer 2

You have duplicates in your tables. The one of interest is station_temporar_con_station.
To find the duplicates, use:
SELECT coordinates_text, MIN(connection_coord_city_type), MAX(connection_coord_city_type)
FROM station_temporar_con_station
GROUP BY coordinates_text;
Then you need to figure out what to do. I would suggest fixing the data.
If you just want to get any matching row in the query, you can use window functions:
SELECT aqt.*, stcs.*
FROM air_quality_temporar aqt LEFT JOIN
(SELECT stcs.*,
ROW_NUMBER() OVER (PARTITION BY coordinates_text ORDER BY coordinates_text) as seqnum
FROM station_temporar_con_station
) stcs
ON stcs.coordinates_text = aqt.coordinates_text AND
stcs.seqnum = 1;
Note that this returns an arbitrary row when there are duplicates. I also replaced the JOIN with LEFT JOIN. The duplicate rows might be hiding the fact that some rows have no matches.