Table schema:
CREATE TABLE orders (date_order_placed VARCHAR, order_id VARCHAR); CREATE TABLE shipments (order_id VARCHAR, invoice_number VARCHAR)

Question: What is the placement date of the order whose invoice number is 10?
SQL: SELECT T1.date_order_placed FROM orders AS T1 JOIN shipments AS T2 ON T1.order_id = T2.order_id WHERE T2.invoice_number = 10