3.30.0 - Support the use of unions in subqueries

What's Changed

Support the use of unions in subqueries by @gwynne in #178

Adds support for the use of UNION queries within subqueries. Unfortunately, thanks to iffy design choices on my part in the original SQLUnion implementation, the usage is slightly awkward. Example usage:

try await db.update("foos")
    .set(SQLIdentifier("bar_id"), to: SQLSubquery
        .union { $0
            .column("id")
            .from("bars")
            .where("baz", .notEqual, "bamf")
        }
        .union(all: { $0
            .column("id")
            .from("bars")
            .where("baz", .equal, "bop")
        })
        .finish()
    )
    .run()

This generates the following query:

UPDATE "foos"
SET "bar_id" = (
        SELECT "id" FROM "bars" WHERE "baz" <> "bamf"
    UNION ALL
        SELECT "id" FROM "bars" WHERE "baz" = "bop"
)

Unfortunately, it is not possible to chain .union() when using SQLSubquery.select(_:); the call chain must start with SQLSubquery.union(_:).

Reviewers

Thanks to the reviewers for their help:

• @MahdiBM

This patch was released by @gwynne

Full Changelog: 3.29.3...3.30.0