WP_Site_Query does not have a $join query clause to compact

[johnjamesjacoby]

The new WP_Site_Query class tries to compact() the join variable, which will never exist because there are no related tables to join against.

Recommend we either:

• Add join clause & $join = ''; to get_site_ids()
• Remove join from compact() statement

[johnjamesjacoby]

Add

[johnjamesjacoby]

Remove

[johnjamesjacoby]

Related: #37923

[flixos90]

I would say we add it in as empty as developers filtering the clauses may expect the key to exist. It also looks a bit more future-proof that way.

[flixos90]

This can be committed, using the first patch.

[jeremyfelt]

I think we can follow what we're doing in WP_Network_Query and make $join available to the sites_clauses filter (where it could be used) but not yet add it to the sql_clauses property (where it wouldn't yet be used).

[jeremyfelt]

In 38631:

Multisite: Provide $join as a possible SQL clause to the sites_clauses filter.

Previously, compact() provided a non existent $join and could cause confusion for anyone attempting to extend WP_Site_Query with their own tables. This aligns with the current behavior in WP_Network_Query.

Props johnjamesjacoby.
Fixes #37922.

[johnjamesjacoby]

Good catch on not needing to add join to $sql_clauses. Thanks @jeremyfelt!