Question

I’ve been struggling with this selection for a while now, I’ve been searching around StackOverflow and tried a bunch of stuff but nothing that helps me with my particular issue. Maybe I’m just missing something obvious.

I have two tables: Measurements, MeasurementFlags

“Measurements” contain measurements from a device, and each measurement can have properties/attributes attached to them (commonly known as “flags”) to signify that the measurement in question is special in some way or another (for instance, one flag may signify a test or calibration measurement). Note: One record per flag!

Right, so a record from the “Measurements” table can theoreticly have an unlimited amount of MeasurementFlags attached to it, or it can have none.

Now, I need to select records from “Measurements”, that have an attached “MeasurementFlag” valued “X”, but it must also NOT have a flag valued “Y” attached to it.

We’re talking about a fairly large database with hundreds of millions of rows, which is why I’m trying to keep all of this logic within one query. Splitting it up would create too many queries, however if it’s not possible to do in one query I guess I don’t have a choise.

Thanks in advance.

Answer 1

SELECT measurements.*
FROM   measurements
INNER JOIN measurement_flag
        ON measurements.measurement_id = measurement_flag.measurement_id
       AND flag = 'X'
       AND 'Y' NOT IN (
               SELECT flag
               FROM   measurement_flag
               WHERE  measurement_id = measurements.measurement_id);