Question

The following query works:

SELECT 
    SUM(sales), 
    date
FROM 
    sales
    INNER JOIN
    exchange_rates
    USING(date) 

But this fails:

SELECT 
    SUM(sales), 
    date
FROM 
    sales
    INNER JOIN
    exchange_rates
    ON sales.date = exchange_rates.date

Why does this fail — I thought they were the same? How would I fix the second query?

Answer 1

In general, if you use the USING clause, the column you are joining on does not need to be aliased in the SELECT list. The database knows that the column is not ambiguous– there is no potential that sales.date will be different than exchange_rates.date. If you use the ON clause, on the other hand, there is the potential for ambiguity. Obviously, in this case, you the human knows that there is no ambiguity, it’s just that the syntax doesn’t guarantee that.

SELECT 
    SUM(sales), 
    sales.date
FROM 
    sales
    INNER JOIN
    exchange_rates
    ON sales.date = exchange_rates.date
GROUP BY sales.date

should work (I’m adding the GROUP BY as well just to be explicit)