I am looking at a practice test that doesn't have explanations about the correct answers. The question I'm confused about basically asks why the following SQL statement can never work:
SELECT oi.order_id, product_jd, order_date FROM order_items oi JOIN orders o USING(order_id); The answer it gave was: "The statement would not execute because the column part of the USING clause cannot have a qualifier in the SELECT list"
Can someone elaborate on this? I am pretty stumped.
1 Answer
It's complaining about the oi qualifier:
SELECT oi.order_id, product_jd, order_date ^^^ Oracle does not allow qualifiers in combination with a using join. The clearest way out is using a regular join:
SELECT oi.order_id, product_jd, order_date FROM order_items oi JOIN orders o ON o.order_id = oi.order_id You can also omit the qualifier. The using statement tells Oracle that even though there are two fields called order_id, they are both equal:
SELECT order_id, product_jd, order_date FROM order_items oi JOIN orders o USING(order_id) 
