MySQL 多次使用一个子查询

Question

我只是 SQL 的初学者，所以请不要担心 :)

我有存储字典单词翻译的表。该表有列id、la（语言）、lb（目标语言）、wa（原始词）、wb（翻译词）和一些更无关紧要的列:)。我想显示一个表格概述，其中将是所有语言的列表，其中包含每种语言中的字数加上 SUM 作为总字数的语言的行（所有仅不同单词的计数）。

我写了这个查询：

SELECT `lng`, COUNT(`word`) AS `count` 
FROM (
  SELECT DISTINCT * 
  FROM (
     SELECT DISTINCT `la` AS `lng`, `wa` AS `word` 
     FROM `dict_trans` 
     UNION ALL 
     SELECT DISTINCT `lb` AS `lng`, `wb` AS `word` 
     FROM `dict_trans`
  ) AS `tbla`
) AS `tblb` GROUP BY `lng`
UNION ALL
SELECT 'sum' AS `lng`, COUNT(`word`) AS `count` 
FROM (
  SELECT DISTINCT * 
  FROM (
    SELECT DISTINCT `la` AS `lng`, `wa` AS `word` 
    FROM `dict_trans` 
    UNION ALL 
    SELECT DISTINCT `lb` AS `lng`, `wb` AS `word` 
    FROM `dict_trans`
  ) AS `tblc`
) AS `tbld` ORDER BY `count` DESC

但我认为多次执行一个子查询非常愚蠢且性能不友好。

SELECT DISTINCT * 
FROM (
  SELECT DISTINCT `la` AS `lng`, `wa` AS `word` 
  FROM `dict_trans` 
  UNION ALL 
  SELECT DISTINCT `lb` AS `lng`, `wb` AS `word` 
  FROM `dict_trans`
) AS `tbla`

我尝试在代码的第二部分从第一部分传递对表的引用：

SELECT `lng`, COUNT(`word`) AS `count` 
FROM (
  SELECT DISTINCT * 
  FROM (
     SELECT DISTINCT `la` AS `lng`, `wa` AS `word` 
     FROM `dict_trans` 
     UNION ALL 
     SELECT DISTINCT `lb` AS `lng`, `wb` AS `word` 
     FROM `dict_trans`
   ) AS `tbla`
) AS `tblb` GROUP BY `lng`
UNION ALL
SELECT 'sum' AS `lng`, COUNT(`word`) AS `count` 
FROM `tblb` 
ORDER BY `count` DESC

但是抛出了错误（#1146 - 表 'db.tblb' 不存在）。

不创建临时表可以解决这个问题吗？

Answer 1

您不需要查询的所有复杂性。 UNION 表示 UNION DISTINCT，因此不需要 3 级嵌套，可以使用 WITH ROLLUP 修饰符完成额外的计数：

SELECT lng AS language, 
       COUNT(word) AS WordCount 
FROM
  ( SELECT la AS lng, wa AS word 
    FROM dict_trans 
  UNION                                 -- DISTINCT is the default
    SELECT lb, wb 
    FROM dict_trans
  ) AS t
GROUP BY lng
  WITH ROLLUP ;

如果你想排序，你需要另一个嵌套：

SELECT language, WordCount 
FROM
  ( SELECT COALESCE(lng, 'Total') AS language, 
           COUNT(word) AS WordCount 
    FROM
      ( SELECT la AS lng, wa AS word 
        FROM dict_trans 
      UNION                                 -- DISTINCT is the default
        SELECT lb, wb 
        FROM dict_trans
      ) AS t
    GROUP BY lng
      WITH ROLLUP 
  ) AS tmp
ORDER BY CASE WHEN language = 'Total' THEN 1 ELSE 0 END,
         WordCount DESC ;

这个查询最昂贵的部分将是UNION（因为您需要DISTINCT）和GROUP BY 操作。如果您想要效率，最好有一个包含所有不同语言和单词组合的单独表，然后在该表上分组（不需要联合）。 dict_trans 将是一个“联结”表，您将有 2 个外键，指向该新的不同语言单词表。