如何混合 COUNT() 和非 COUNT() 列而不丢失查询中的信息？

Posted 2023-03-22

【中文标题】如何混合 COUNT() 和非 COUNT() 列而不丢失查询中的信息？

【英文标题】：How can I mix COUNT() and non-COUNT() columns without losing information in the query?

【发布时间】：2009-01-13 16:26:21

【问题描述】：

我从一个查询开始：

SELECT strip.name as strip, character.name as character
  from strips, characters, appearances
 where strips.id = appearances.strip_id
   and characters.id = appearances.character.id
   and appearances.date in (...)

这给了我一些结果：

strip                 | character
'Calvin & Hobbes'     | 'Calvin'
'Calvin & Hobbes'     | 'Hobbes'
'Pearls Before Swine' | 'Pig'
'Pearls Before Swine' | 'Rat'
'Pearls Before Swine' | 'Hobbes'  # a guest appearance
'Pearls Before Swine' | 'Calvin'  # a guest appearance

然后我还想获得COUNT 在结果集中（在任何条带中）使用字符的次数。所以我尝试了：

SELECT count(character.id), strip.name as strip, character.name as character
  from strips, characters, appearances
 where strips.id = appearances.strip_id
   and characters.id = appearances.character.id
   and appearances.date in (...)

但这给了我

[ERROR 11:20:17] Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause

所以我尝试了：

SELECT count(character.id), strip.name as strip, character.name as character
  from strips, characters, appearances
 where strips.id = appearances.strip_id
   and characters.id = appearances.character.id
   and appearances.date in (...)
 group by character.id

这给了我

count | strip                 | character
4     | 'Calvin & Hobbes'     | 'Calvin'
4     | 'Calvin & Hobbes'     | 'Hobbes'
2     | 'Pearls Before Swine' | 'Pig'
2     | 'Pearls Before Swine' | 'Rat'

也就是说，我丢失了关于哪些字符出现在哪些条带中的所有额外信息。

我想要的是这个：

count | strip                 | character
4     | 'Calvin & Hobbes'     | 'Calvin'
4     | 'Calvin & Hobbes'     | 'Hobbes'
2     | 'Pearls Before Swine' | 'Pig'
2     | 'Pearls Before Swine' | 'Rat'
4     | 'Pearls Before Swine' | 'Calvin'
4     | 'Pearls Before Swine' | 'Hobbes'

但我似乎无法弄清楚。如果重要的话，我在 mysql 上。也许只需要两个查询。

【参考方案1】：

mySQL 是否支持分析函数？喜欢：

SELECT foo.bar, baz.yoo, count(baz.yoo) over (partition by foo.bar) as yoo_count 
from foo, bar
where foo.baz_id = baz.id and baz.id in (...)

或者：

SELECT foo.bar, baz.yoo, v.yoo_count 
from foo, bar, 
( select foo.baz_id, count(*) as yoo_count
  from foo
  group by foo.baz_id
) as v
where foo.baz_id = baz.id and baz.id in (...)
and v.baz_id = foo.baz_id;

【讨论】：

MySQL 不做分区，后者给我错误信息“子查询返回超过 1 行。”

【参考方案2】：

按 foo.bar 分组怎么样？

SELECT count(baz.id) as count, foo.bar, baz.yoo where foo.baz_id = baz.id and baz.id in (...) group by foo.bar

【参考方案3】：

在 RDBMS 系统上，我会将计数查询选择到临时表中，然后将临时表重新加入主表。这将为您提供两全其美的体验。

不过，我不知道 mySQL 是否支持临时表。

【参考方案4】：

如果 MySQL 支持分析/窗口函数，那么：

select bar, yoo,
    count(yoo) over (partition by yoo) c 
from t
/

否则您需要使用相关子查询：

select bar, yoo,
    (select count(yoo) from t t2 where t2.yoo=t.yoo) c 
from t
/

在 oracle 上，测试数据如下：

create table t(bar number, yoo varchar2(16));

insert into t(bar, yoo) values (1, 'hello');

insert into t(bar, yoo) values (2, 'goodbye');

insert into t(bar, yoo) values (3, 'goodbye');

insert into t(bar, yoo) values (4, 'goodbye');

insert into t(bar, yoo) values (2, 'calvin');

insert into t(bar, yoo) values (5, 'Hobbes');

insert into t(bar, yoo) values (6, 'Hobbes');

commit;

【参考方案5】：

子选择：

SELECT foo.bar
    ,baz.yoo
    ,(SELECT COUNT(*) FROM baz AS baz2 WHERE baz2.yoo = baz.yoo etc.) AS yoo_count
FROM foo, baz
WHERE foo.baz_id = baz.id
    AND baz.id in (...)

您也可以将子选择嵌套并加入。

【讨论】：

不，这给了我每行返回的所有行的总数。

【参考方案6】：

如果我要经常需要这样的计数字段，那么我会创建一个用户定义的函数来返回值并在普通选择中使用它。