优化mysql查询:奖章排名

Question

我有两张桌子：

1. 列olympic_medalists ( gold_country、silver_country、bronze_country )
2. 带国家栏的旗帜

我想相应地列出奥运奖牌表。我有这个查询，它可以工作，但它似乎扼杀了mysql。希望有人能帮助我进行优化查询。

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)

结果如下：

country  |  g  |  s  |  b  |  tot
---------------------------------
country1 |  9  |  5  |  2  |  16
country2 |  5  |  5  |  5  |  15

诸若此类

谢谢!

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

这将比当前为交叉连接关系中的每一行执行三个不同的SELECT子查询的解决方案高效得多(您想知道为什么会停下来！)：

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

更快的不是将实际的文本国家名称存储在每个金、银和青铜列中，而是存储基于整数的country id。对整数的比较总是比字符串上的比较快。

此外，一旦您用相应的id替换了olympic_medalists表中的每个国家名称，您将希望在每一列(黄金、银和青铜)上创建一个索引。

将文本名称更新为相应的id是一项简单的任务，可以使用单个UPDATE语句和一些ALTER TABLE命令来完成。

Answer 2

试试这个：

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