Overhaul schema command, remove database name

[rgwood]

This PR changes the schema command for viewing the schema of a SQLite database file. It removes 1 level of nesting (intended to handle multiple databases in the same connection) that I believe is unnecessary.

Before

After

Rationale

A SQLite database connection can technically be associated with multiple non-temporary databases using the ATTACH DATABASE command. But it's not possible to do that in the context of Nushell, and so I believe that there is no benefit to displaying the schema as if there could be multiple databases.

I initially raised this concern back in April, but we decided to keep the database nesting because at the time we were still looking into more generalized database functionality (i.e. not just SQLite). I believe that rationale no longer applies.

Also, the existing code would not have worked correctly even if a connection had multiple databases; for every database, it was looking up tables without filtering them by database:

nushell/crates/nu-command/src/database/values/sqlite.rs

Lines 104 to 118 in 6295b20

	pub fn get_databases_and_tables(&self, conn: &Connection) -> Result<Vec<Db>, rusqlite::Error> {
	let mut db_query = conn.prepare("SELECT name FROM pragma_database_list")?;
	let databases = db_query.query_map([], |row| {
	let name: String = row.get(0)?;
	Ok(Db::new(name, self.get_tables(conn)?))
	})?;
	let mut db_list = vec![];
	for db in databases {
	db_list.push(db?);
	}
	Ok(db_list)
	}

Future Work

I'd like to add information on views+triggers to the schema output. I'm also working on making it possible to ctrl+c reading from a database (which is turning into a massive yak shave).

[rgwood]

@fdncred are you OK with this change?

[fdncred]

I like being able to see the file name but I can live with your changes.

[rgwood]

Thanks. My thinking was that the user probably knows the file name already (they probably typed open filename | schema) and it's not really part of the schema.

Maybe later I can upgrade describe so it includes the file path? Currently open foo.db | describe just returns SQLiteDatabase