Update min Swift version to 5.6 and make platform versions consistent

This patch was authored and released by @gwynne.

Make SQLList's properties mutable

This patch was authored and released by @gwynne.

Improves consistency with other SQLKit types (and is incidentally convenient for Fluent).

Support += operator for SQLQueryString.

This patch was authored by @maciejtrybilo and released by @gwynne.

Very simple change to support the += operator for appending SQLQueryStrings.

Bump minimum Swift version to 5.5

This patch was authored and released by @gwynne.

Also syncs up CI with the latest refinements.

Add support for varying syntax of SELECT locking clauses.

This patch was authored and released by @gwynne.

The FOR SHARE syntax previously provided by adding .for(.share) to a SELECT query is specific to PostgreSQL. This update allows each database driver to specify the correct syntax for locking clauses - or to signal that it doesn't implement that functionality - via its SQLDialect. The default is to assume locking clauses are not supported.

Because new public API is added to SQLStatement and SQLDialect, this update is semver-minor.

PR for MySQL support: vapor/mysql-kit#310
PR for PostgreSQL support: vapor/postgres-kit#230
PR for SQLite support: <not required, SQLite does not support locking clauses>

Adds `LIMIT`, `OFFSET` and `ORDER BY` to Union results

This patch was authored by @NeedleInAJayStack and released by @gwynne.

Previously it was not easy to do pagination on the result of SELECT ... UNION queries. For example:

(SELECT * FROM "Table" WHERE "name" = 'first thing')
UNION ALL
(SELECT * FROM "Zone" WHERE "name" = 'second thing')
LIMIT 5
OFFSET 3
ORDER BY "name"

This pull request adds LIMIT, OFFSET, and ORDER BY functionality to the SQLUnion/SQLUnionBuilder, primarily by copying over the SQLSelect and SQLSubqueryClauseBuilder implementations.

I also have a protocol-based approach that reduces code copying but it comes with it's own smells. I'm happy to elaborate if desired.

UNION handles single entry

This patch was authored by @NeedleInAJayStack and released by @gwynne.

This PR simplifies dynamically building UNION statements.

To be specific, it loosens the restriction that a SQLUnion must contain multiple select statements. While this is common usage, it makes building up UNIONs in client code difficult. For example, building up a UNION in a for loop is awkward right now:

let ids = [1, 2, 3, ...]
guard let firstId = ids.first else { ... }

// Must manually short-circuit as a SQLSelectBuilder
guard ids.count > 1 else {
    return sql.select.column("id").from("t1").where("id", .equals, firstId).all()
}
let unionBuilder = sql.union { select in
    select.column("id").from("t1").where("id", .equals, firstId)
}
for id in ids[1..<ids.count] {
    unionBuilder.union(all: { select in
        select.column("id").from("t1").where("id", .equals, id)
    })
}
return unionBuilder.all()

This PR removes the need for the commented guard in the code above. It also improves code safety by removing a runtime fatal error condition.

Add ability for SQLKit drivers to expose version numbers

This patch was authored and released by @gwynne.

Also fills in some missing documentation.

Add support for MySQL's DROP INDEX syntax

This patch was authored and released by @gwynne.

MySQL's version of DROP INDEX differs from PostgreSQL's in that it requires an ON <table name> clause in order to identify the index to drop, since MySQL treats indexes as table-level objects rather than schema-level. This PR adds support for optionally specifying the table name.

Also deprecates SQLDataType.type(_:), which should never have been public - it is a utility method used in one place by a single test case; its behavior is only valid in PostgreSQL, and even then only for custom-typed enums, but its name and placement give no hint of this to callers. Since it is public API, it can not be summarily removed without a semver-major break, but we can at least remove the single call site and discourage any further use.

Also updates the CI workflows as per the usual in my PRs 🙂.

Note: semver-minor not only due to the new public API on SQLDropIndexBuilder but also because the deprecation of a public API is, while not source-breaking, annoying for anyone who actually used it.

Adds `ALTER _ RENAME TO _` support

This patch was authored by @NeedleInAJayStack and released by @0xTim.

Adds table-renaming support to SQLAlterTableBuilder. This produces SQL queries of the form:

ALTER `table_name` RENAME TO `new_table_name`