While I was working on our survey sample characterisation paper, I had to redo the sample selection again. There's an interesting effect in diameter limiter surveys: they should theoretically include more highly-inclined disk galaxies near the lower diameter limit, as their total brightness is projected onto a smaller area.
I was selecting two samples from the SDSS DR7: one which was only redshift-selected, and another which had no cuts in redshift, but was diameter-limited. I wanted to keep the redshift information for the diameter-limited sample (wherein not all galaxies have spectroscopic redshifts), if it was available for a particular galaxy. Hence, outer joins.
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