Fetch flake outputs and display them

schema.sql

@@ -1,20 +1,20 @@
 CREATE TABLE revisions (
 	revision_uri TEXT NOT NULL PRIMARY KEY,
 	flake_uri TEXT NOT NULL,
-	store_path TEXT,
-	last_modified INT
+	store_path TEXT NOT NULL,
+	last_modified INT NOT NULL
 );
 
 CREATE TABLE output_attributes (
 	revision_uri TEXT NOT NULL,
 	output_attribute_name TEXT NOT NULL,
-	derivation_path TEXT,
+	derivation_path TEXT NOT NULL,
 	PRIMARY KEY (revision_uri, output_attribute_name)
 );
 
 CREATE TABLE build_outputs (
 	derivation_path TEXT NOT NULL,
 	build_output_name TEXT NOT NULL,
-	store_path TEXT,
+	store_path TEXT NOT NULL,
 	PRIMARY KEY (derivation_path, build_output_name)
 );