How can I optimize a query involving a subselect in MySQL? - mysql

Question

Suppose I have a table like this:
id | price | group1
1 | 6 | some_group
2 | 7 | some_group
3 | 8 | some_group
4 | 9 | some_other_group
If I want to select the lowest price grouped by group1
I can just do this:
SELECT id, min(price), group1 FROM some_table GROUP BY group1;
The problem is when I have a table which is not sorted by price like this:
id | price | group1
1 | 8 | some_group
2 | 7 | some_group
3 | 6 | some_group
4 | 9 | some_other_group
Then my query returns this result set:
id | price | group1
1 | 6 | some_group
4 | 9 | some_other_group
The problem is that I get 1 in the id column but the id of the row with the price of 6 is not 1 but 3.
My question is that how can I get the values from the row which contains the minimum price when I use GROUP BY?
I tried this:
SELECT f.id, min(f.price), f.group1 FROM (SELECT * FROM some_table ORDER BY price) f
GROUP BY f.group1;
but this is really slow and if I have multiple columns and aggregations it may fail.
Please note that the names above are just for demonstration purposes. My real query looks like this:
SELECT depdate, retdate, min(totalprice_eur) price FROM
(SELECT * FROM flight_results
WHERE (
fromcity = 30001350
AND tocity = 30001249
AND website = 80102118
AND roundtrip = 1
AND serviceclass = 1
AND depdate > date(now()))
ORDER BY totalprice_eur) F
WHERE (
fromcity = 30001350
AND tocity = 30001249
AND website = 80102118
AND roundtrip = 1
AND serviceclass = 1
AND depdate > date(now()))
GROUP BY depdate,retdate
and there is a concatenated primary key including website, fromcity, tocity, roundtrip, depdate, and retdate. There are no other indexes.
Explain says:
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 2837 Using where; Using temporary; Using filesort
2 DERIVED flight_results ALL PRIMARY NULL NULL NULL 998378 Using filesort

Answer 1

You can do this instead:
SELECT t1.id, t1.price, t1.group1
FROM some_table AS t1
INNER JOIN
(
SELECT min(price) minprice, group1
FROM some_table
GROUP BY group1
) AS t2 ON t1.price = t2.minprice AND t1.group1 = t2.group1;