Added patch file for database

diff.sql

+/*
+ * This file is to patch up an already existing SQLite file
+ * to use the new product diffing system
+ */
+CREATE TABLE diff_help (
+    id INTEGER PRIMARY KEY NOT NULL,
+    product_id INTEGER NOT NULL,
+    amount INTEGER,
+
+    FOREIGN KEY (product_id) REFERENCES products(id)
+);
+
+CREATE VIEW diff_view AS
+SELECT d.id AS id,
+       p.id AS product_id,
+       p.sale_status AS sale_status,
+       -- visible border
+       p.name AS name,
+       s.active AS expected,
+       d.amount AS actual,
+       d.amount - s.active AS diff
+FROM products p
+INNER JOIN stock s ON p.id = s.product_id
+INNER JOIN diff_help d ON p.id = d.product_id;
+
+CREATE TRIGGER after_diff_help_delete_last
+AFTER DELETE ON diff_help
+FOR EACH ROW
+WHEN (SELECT count (1) = 0 FROM diff_help)
+BEGIN
+    INSERT INTO diff_help (product_id)
+    SELECT id FROM products p;
+END;
+
+CREATE TRIGGER diff_view_transfer
+INSTEAD OF DELETE ON diff_view
+BEGIN
+    UPDATE stock SET active = OLD.actual
+    WHERE OLD.actual IS NOT NULL
+      AND product_id = OLD.product_id;
+
+    INSERT INTO stock_diff (product_id, expected, actual)
+    SELECT OLD.product_id, OLD.expected, OLD.actual
+     WHERE OLD.actual IS NOT NULL;
+
+    DELETE FROM diff_help WHERE id = OLD.id;
+END;
+
+INSERT INTO diff_help (product_id)
+SELECT id FROM products LIMIT 1;
+DELETE FROM diff_help;