Question

I found this example on the MySQL Tutorial:

SELECT article, dealer, price
FROM   shop
WHERE  price=(SELECT MAX(price) FROM shop);

My question: is the subquery (SELECT MAX(price) FROM shop) done once a time, or it is done repeatedly until the max price for the query is found?

In terms of performance is this other solution better?

SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.price < s2.price
WHERE s2.article IS NULL;

Thanks.

Answer 1

The sub query is non correlated so any sensible implementation will only evaluate it once. Note that MySQL does have a problem with IN though where the semantically equivalent

SELECT article, dealer, price
FROM   shop
WHERE  price IN (SELECT MAX(price) FROM shop);

Leads to the sub query being evaluated multiple times.

As far as evaluating performance you would need to look at the explain plan for both in your particular RDBMS.

The most efficient solution might be to use SELECT TOP .. WITH TIES or equivalent if you have a covering index on the price column and your RDBMS has such a construct.