The tricky point in ADQL is the FROM clause. So far, we had a single table. Things get interesting when you add more tables: JOIN.

SELECT TOP 10 lat, long, flux
FROM lightmeter.measurements
JOIN lightmeter.stations
USING (stationid)
Check the tables in the Table Metadata shown by TOPCAT: flux is from measurements, lat and long from stations; both tables have a stationid column.

JOIN is a combination of cartesian product and a select.

measurements JOIN stations USING (stationid)

yields the cartesian product of the measurement and stations tables but only retains the rows in which the stationid columns in both tables agree.

Note that while the stationid column we’re joining on is in both tables but only occurs once in the joined table.

Markus Demleitner

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