Question

我正在尝试查询数据库，我认为我的查询可能会缩短以消除子查询，但我这样做的每次尝试都会导致某种错误（我对SQL有点新）。

该数据库按年龄和性别群体提供有关学校人口统计数据的信息。我试图通过以下查询获取有关性别的摘要信息：

SELECT i.id AS id,
  (SELECT group_concat(entry separator '`')
   FROM
     (SELECT school_id,
             concat(gender, '|', sum(percentage)) AS entry
      FROM school_demographics
      GROUP BY gender,
               school_id) AS s1
   WHERE s1.school_id = i.id ) AS demographics
FROM school_info i
WHERE i.id = 500;

这样可行，并将我需要的所有内容合并到一个female|65.4`male|34.4形式的字符串中（正如我所愿），但我有这种感觉，不需要其中一个子查询，特别是我返回列AS entry只在上面的group_concat中使用它。我是否认为有一种更简单的方法可以使此查询有效？

Answer 1

结果必须为每个查询的id
返回一行

这并不意味着，它不能返回多个列。因此，您可以尝试条件聚合（假设female列中只有male和gender）：

SELECT school_id,
    SUM(CASE gender WHEN 'female' THEN percentage END) as percentage_female,
    SUM(CASE gender WHEN 'male'   THEN percentage END) as percentage_male
FROM school_demographics
GROUP BY school_id
WHERE school_id = 500

结果如下：

school_id | percentage_female | percentage_male
      500 |              65.4 |            34.4

Answer 2

请注意，彼此连接的子查询可能性能较差

select i.id,group_concat(d.gender,'|',d.p separator '`')
from
 (select school_id,gender,sum(percentage) as p
  from school_demographics
  group by school_id,gender
  ) d
  join school_info i on i.id=d.school_id
group by i.id
where i.id=500;

where i.id=500可以删除

我是否创建了一个我不需要的MySQL表？

2 个答案: