SQL - 按不同值分组

我的问题是否有更快的方式来查询以下内容？SQL - 按不同值分组

我使用Oracle 10g

说我有一个表，制造商和汽车，我要算列“Car.Name”的所有事件。这里是我会怎么做它：

SELECT manuf.Name, COUNT(car1.Name), COUNT(car2.Name), COUNT(car3.Name) 
FROM Manufacturer manuf 
LEFT JOIN (SELECT * FROM Car c where c.Name = 'Ferrari1') car1 ON manuf.PK = car1.ManufPK 
LEFT JOIN (SELECT * FROM Car c where c.Name = 'Ferrari2') car2 ON manuf.PK = car2.ManufPK 
LEFT JOIN (SELECT * FROM Car c where c.Name = 'Ferrari3') car3 ON manuf.PK = car3.ManufPK 
GROUP BY manuf.Name

通缉结果：

Manufacturer | Ferrari1 | Ferrari2 | Ferrari3 
---------------------------------------------- 
Fiat   | 1  | 0  | 5 
Ford   | 2  | 3  | 0

我很少LEFT JOIN的尝试这样做，它工作得很好。但是当我添加了很多（如90+）时，它超慢（超过1分钟）。

我的问题，有没有更快的方法来做这个查询？

来源

2012-01-25 WoF_Angel

你能告诉我们什么是你以前的查询？ – everton

@WoF_Angel您使用哪种RDBMS？在MySQL中，您可以考虑在MSSQL中使用'GROUP_CONCAT'关于'PIVOT' –

[通过正确的索引，多个连接可能至少会像PIVOT/GROUP BY一样执行]（http://stackoverflow.com/questions/ 7448453/sql-server-pivot-vs-multiple-join/7449213＃7449213） –

如果你很高兴看到汽车倒计时页面，请尝试：

select m.Name manufacturer_name, 
     c.Name car_name, 
     count(*) 
from Manufacturer m 
left join Car c 
     on m.PK = c.ManufPK and c.Name in ('Ferrari1','Ferrari2','Ferrari3') 
group by m.Name, c.Name

如果你需要看到整个页面各个汽车，尝试：

select m.Name manufacturer_name, 
     sum(case c.Name when 'Ferrari1' then 1 else 0 end) Ferrari1_Count, 
     sum(case c.Name when 'Ferrari2' then 1 else 0 end) Ferrari2_Count, 
     sum(case c.Name when 'Ferrari3' then 1 else 0 end) Ferrari3_Count 
from Manufacturer m 
left join Car c 
     on m.PK = c.ManufPK and c.Name in ('Ferrari1','Ferrari2','Ferrari3') 
group by m.Name

来源

2012-01-25 11:43:53

他提到过。 – everton

@aF那么是什么？这不适合需求 - 所以我们需要一些细化的问题 –

我用你的第二个建议，工作得很好。谢谢。 –

SELECT manuf.Name, COUNT(DISTINCT c.Name) 
FROM Manufacturer manuf 
LEFT JOIN Car c ON manuf.PK = c.ManufPK 
GROUP BY manuf.Name

OR根据您的需要

SELECT manuf.Name, c.Name, COUNT(*) Cnt 
FROM Manufacturer manuf 
LEFT JOIN Car c ON manuf.PK = c.ManufPK 
GROUP BY manuf.Name, c.Name

PS：你的问题不是很清楚。提供了一些有用的结果集细化答案

来源

2012-01-25 11:42:20

这不会给出同一行中的所有计数。 –

@MarkBannister查看评论回复 –

你也可以试试这个：

SELECT manuf.Name 
    , car1.cnt AS Ferrari1 
    , car2.cnt AS Ferrari2 
    , car3.cnt AS Ferrari3 
FROM 
     Manufacturer AS manuf 
    LEFT JOIN 
     (SELECT ManufPK, COUNT(*) AS cnt 
     FROM Car 
     WHERE Name = 'Ferrari1' 
     GROUP BY ManufPK 
    ) AS car1 
    ON car1.ManufPK = manuf.PK 
    LEFT JOIN 
     (SELECT ManufPK, COUNT(*) AS cnt 
     FROM Car 
     WHERE Name = 'Ferrari2' 
     GROUP BY ManufPK 
    ) AS car2 
    ON car2.ManufPK = manuf.PK 
    LEFT JOIN 
     (SELECT ManufPK, COUNT(*) AS cnt 
     FROM Car 
     WHERE Name = 'Ferrari3' 
     GROUP BY ManufPK 
    ) AS car3 
    ON car3.ManufPK = manuf.PK 
ORDER BY manuf.Name

来源

2012-01-25 13:07:49

SQL - 按不同值分组

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