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f05a5e0003
Commit fede15417 introduced FILTER by jamming it into the existing example introducing HAVING, which seems pedagogically poor to me; and it added no information about what the keyword actually does. Not to mention that the claimed output didn't match the sample data being used in this running example. Revert that and instead make an independent example using FILTER. To help drive home the point that it's a per-aggregate filter, we need to use two aggregates not just one; for consistency expand all the examples in this segment to do that. Also adjust the example using WHERE ... LIKE so that it'd produce nonempty output with this sample data, and show that output. Back-patch, as the previous patch was. (Sadly, v10 is now out of scope.) Discussion: https://postgr.es/m/166794307526.652.9073408178177444190@wrigleys.postgresql.org
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