`meta_compare_key` should support more operators than `LIKE` and `=`

[boonebgorges]

meta_compare_key was introduced in #42409. The default is = and it also supports LIKE. It could potentially support more options.

See https://core.trac.wordpress.org/ticket/42409#comment:4 and especially https://core.trac.wordpress.org/ticket/42409#comment:13 for some important considerations.

[soulseekah]

!=, NOT LIKE, IN, NOT IN

[soulseekah]

Tests for !=, NOT LIKE, IN, NOT IN

[soulseekah]

Started on this.

Implemented the following operators: !=, IN, NOT IN, NOT LIKE.

The numerical operators don't make too much sense, I think, since nobody names meta keys with pure numbers out there... I hope :)

I've marked EXISTS, NOT EXISTS (these are pretty much = and != operators as is, they can probably be aliased), RLIKE, REGEXP and NOT REGEXP for later, so just looking to get some eyes on what I have so far.

Special thanks to @versusbassz for working with me on this. He'll upload the modified docblock once all operators are covered.

Cheers.

[soulseekah]

!=, NOT LIKE, IN, NOT IN, EXISTS, NOT EXISTS, REGEXP, NOT REGEXP, RLIKE

[soulseekah]

Tests for !=, NOT LIKE, IN, NOT IN, EXISTS, NOT EXISTS, REGEXP, NOT REGEXP, RLIKE

[soulseekah]

Added EXISTS, NOT EXISTS, REGEXP, NOT REGEXP, RLIKE implementations.

Docbloc changes incoming...

[soulseekah]

Docblock

[SergeyBiryukov]

Assigning to mark the good-first-bug as "claimed."

[pento]

Patches need review.

[desrosj]

Latest patch still applies cleanly and needs the @since tags updated.

@boonebgorges are you able to review this in time for 5.2?

Also, marking as needs-dev-note so one can be drafted once the change is merged.

[boonebgorges]

Sorry for missing this when the patches came in. I don't have time to review for the 5.2 deadline, but I'll take a look for 5.3.

[boonebgorges]

@soulseekah - Thanks so much for your work on this patch. I've reviewed it and I think it's strong. 43446.diff​ makes a few code formatting and documentation changes.

I'd like to propose that we do not include EXISTS and NOT EXISTS. The semantics of EXISTS suggests to me something less specific than = - as in the case of key = foo && compare = EXISTS, where you're omitting the value - but there's no corresponding "less specific" query in the case of keys. All rows in the meta table have a key. It just feels a little confusing to me. Perhaps others don't share my intuition - I'm happy to be overruled. In that case, I might make the = and != aliases clearer in the documentation.

This should be doable for 5.3, but I'll hold off a week or two in case someone wants to review or comment before the beta.

[boonebgorges]

Fixed in [46188].

[justinahinon]

Mentioned in ​https://make.wordpress.org/core/2019/10/15/expanded-meta-key-comparison-operators-in-5-3

props @boonebgorges for the note