optimize mysql query: medal standings - mysql

Question

I have 2 tables:
olympic_medalists with columns gold_country, silver_country, bronze_country
flags with country column
I want to list the olympic medal table accordingly. I have this query, it works, but it seems to kill mysql. Hope someone can help me with an optimized query.
SELECT DISTINCT country AS sc,
IFNULL(
(SELECT COUNT(silver_country)
FROM olympic_medalists
WHERE silver_country = sc AND silver_country != ''
GROUP BY silver_country),0) AS silver_medals,
IFNULL(
(SELECT COUNT(gold_country)
FROM olympic_medalists
WHERE gold_country = sc AND gold_country != ''
GROUP BY gold_country),0) AS gold_medals,
IFNULL(
(SELECT COUNT(bronze_country)
FROM olympic_medalists
WHERE bronze_country = sc AND bronze_country != ''
GROUP BY bronze_country),0) AS bronze_medals
FROM olympic_medalists, flags
GROUP BY country, gold_medals, silver_country, bronze_medals HAVING (
silver_medals >= 1 || gold_medals >= 1 || bronze_medals >= 1)
ORDER BY gold_medals DESC, silver_medals DESC, bronze_medals DESC,
SUM(gold_medals+silver_medals+bronze_medals)
result will be like:
country | g | s | b | tot
---------------------------------
country1 | 9 | 5 | 2 | 16
country2 | 5 | 5 | 5 | 15
and so on
Thanks!
olympic medalists:
`id` int(8) NOT NULL auto_increment,
`gold_country` varchar(64) collate utf8_unicode_ci default NULL,
`silver_country` varchar(64) collate utf8_unicode_ci default NULL,
`bronze_country` varchar(64) collate utf8_unicode_ci default NULL, PRIMARY KEY (`id`)
flags
`id` int(11) NOT NULL auto_increment,
`country` varchar(128) default NULL,
PRIMARY KEY (`id`)

Answer 1

This will be much more efficient than your current solution of executing three different SELECT subqueries for each row in a cross-joined relation (and you wonder why it stalls out!):
SELECT a.country,
COALESCE(b.cnt,0) AS g,
COALESCE(c.cnt,0) AS s,
COALESCE(d.cnt,0) AS b,
COALESCE(b.cnt,0) +
COALESCE(c.cnt,0) +
COALESCE(d.cnt,0) AS tot
FROM flags a
LEFT JOIN (
SELECT gold_country, COUNT(*) AS cnt
FROM olympic_medalists
GROUP BY gold_country
) b ON a.country = b.gold_country
LEFT JOIN (
SELECT silver_country, COUNT(*) AS cnt
FROM olympic_medalists
GROUP BY silver_country
) c ON a.country = c.silver_country
LEFT JOIN (
SELECT bronze_country, COUNT(*) AS cnt
FROM olympic_medalists
GROUP BY bronze_country
) d ON a.country = d.bronze_country
What would be even faster is instead of storing the actual textual country name in each of the gold, silver, and bronze columns, just store the integer-based country id. Comparisons on integers are always going to be faster than comparisons on strings.
Moreover, once you replace each country name in the olympic_medalists table with the corresponding id's, you'll want to create an index on each column (gold, silver, and bronze).
Updating the textual names to be their corresponding id's instead is a simple task and could be done with a single UPDATE statement in conjunction with some ALTER TABLE commands.

Answer 2

try this:
SELECT F.COUNTRY,IFNULL(B.G,0) AS G,IFNULL(B.S,0) AS S,
IFNULL(B.B,0) AS B,IFNULL(B.G+B.S+B.B,0) AS TOTAL
FROM FLAGS F LEFT OUTER JOIN
(SELECT A.COUNTRY,
SUM(CASE WHEN MEDAL ='G' THEN 1 ELSE 0 END) AS G,
SUM(CASE WHEN MEDAL ='S' THEN 1 ELSE 0 END) AS S,
SUM(CASE WHEN MEDAL ='B' THEN 1 ELSE 0 END) AS B
FROM
(SELECT GOLD_COUNTRY AS COUNTRY,'G' AS MEDAL
FROM OLYMPIC_MEDALISTS WHERE GOLD_COUNTRY IS NOT NULL
UNION ALL
SELECT SILVER_COUNTRY AS COUNTRY,'S' AS MEDAL
FROM OLYMPIC_MEDALISTS WHERE SILVER_COUNTRY IS NOT NULL
UNION ALL
SELECT BRONZE_COUNTRY AS COUNTRY,'B' AS MEDAL
FROM OLYMPIC_MEDALISTS WHERE BRONZE_COUNTRY IS NOT NULL)A
GROUP BY A.COUNTRY)B
ON F.COUNTRY=B.COUNTRY
ORDER BY IFNULL(B.G,0) DESC,IFNULL(B.S,0) DESC,
IFNULL(B.B,0) DESC,IFNULL(B.G+B.S+B.B,0) DESC,F.COUNTRY