sql: add assembly table

schema.sql

@@ -44,6 +44,16 @@ CREATE TABLE IF NOT EXISTS part (
 	FOREIGN KEY (package) REFERENCES package (id)
 );
 
+-- a project (as part) can be an assembly of other parts
+CREATE TABLE IF NOT EXISTS assembly (
+	assembled INTEGER NOT NULL, -- the assembly project
+	component INTEGER NOT NULL, -- the component part
+	quantity INTEGER NOT NULL, -- quantity of component
+	FOREIGN KEY (assembled) REFERENCES part (id),
+	FOREIGN KEY (component) REFERENCES part (id),
+	UNIQUE (assembled, component)
+);
+
 -- CAD drawing type
 CREATE TABLE IF NOT EXISTS cad (
 	id INTEGER AUTO_INCREMENT PRIMARY KEY, -- index