从两个表中选择并有条件地折叠一列

Question

I have a list of Products

upc | name     | price | qty
----------------------------
1   | apple    |  1.00 |  3
2   | peach    |  2.00 |  7
3   | melon    |  1.75 |  2

and SaleProducts

upc | price
------------
2   | 1.90

I want to select from Products but also sale price from SaleProducts (if product is on sale). This is what I came up with:

SELECT t1.upc, t1.name, MIN(t1.price) AS 'price', t1.qty
FROM (
  SELECT p.upc, p.name, p.price, p.qty
  FROM products p
    UNION
  SELECT sp.upc, NULL, sp.price, NULL
  FROM saleproducts sp
  ) t1
GROUP BY t1.upc;

Output:

upc | name     | price | qty
----------------------------
1   | apple    |  1.00 |  3
2   | peach    |  1.90 |  7
3   | melon    |  1.75 |  2

Can anyone suggest a more elegant way to accomplish this? Im aware of similar question but my goal is to grab whichever price is lower, so COALESCE wouldn't work.

The only restriction is, I must use vanilla SQL, no stored procs or IF's.

Answer 1

Try this instead using CASE :

SELECT p.upc, p.name,
  CASE WHEN sp.price IS NOT NULL
    THEN CASE WHEN p.price > sp.price
      THEN sp.price
      ELSE p.price
    END
    ELSE p.price
  END price, p.qty
FROM products p LEFT JOIN saleproducts sp ON p.upc = sp.upc;

It will prefer to use the price from saleproducts when it's available. If there is not a sale price for that product it will instead use the price from products .

EDIT - I've updated my answer so that it always gets the lowest price. FWIW I can't imagine why you'd bother having a sale price which is actually higher than your list price.

Answer 2

This seems more like the job for a left outer join :

SELECT p.upc, p.name,
       (case when sp.price is null or p.price < sp.upc  then p.price
             else sp.price
        end) as price,
       p.qty
FROM products p left outer join
     salesproducts sp
     on p.upc = sp.upc;