Fix prev/next expression examples

[intermittentnrg]

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Fixes #[insert issue link, if any]

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[billy-the-fish]

Hi @intermittentnrg, thanks for this work. Is this PR still a draft or are you ready for review? Iain

[intermittentnrg]

Ready for review. I tested & used the updated queries. But in the end couldn't use prev expression because there were no datapoints in queried range and prev expression is only used for datapoints within range.

[billy-the-fish]

Thanks @intermittentnrg , much appreciated.

[jonatas]

Should we add this to the group by clause too?

[intermittentnrg]

Yes good point. GROUP BY device_id, day.

device_id should be either in GROUP BY or WHERE to make query realistic.

[jonatas]

Suggested change

	GROUP BY day
	GROUP BY day, device_id

[jonatas]

Suggested change

	GROUP BY day
	GROUP BY day, device_id