mySQL计数表

基本上我有一个表名myTable，里面有name，Class，我想像下面的表一样对它们进行分类，每个名称的计数和它们拥有的1,2,3个数字。下面是我目前拥有的代码，但它只处理少量的条目，如果表有20000行，则需要很长时间才能执行

Select DISTINT(Name),
       (Select count(*) from `myTable` d1 where d1.Name = dd.Name and Class=1),
       (Select count(*) from `myTable` d2 where d2.Name = dd.Name and Class=2),
       (Select count(*) from `myTable` d3 where d3.Name = dd.Name and Class=3)
from `myTable` dd
Order by Name;

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使用条件计数进行分组：

select Name,
       count(case when class = 1 then 1 end),
       count(case when class = 2 then 1 end),
       count(case when class = 3 then 1 end)
from `myTable`
group by Name;

注意：

DISTINCT

不是列上的函数，它作用于整个选定行。（列名是否放在括号内并不重要。）

与

select distinct col1, col2, ...

select distinct col1, (col2), ...

也和

select distinct col1, col2, ...

select distinct col1, (col2), ...

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这是典型的条件聚合：

Select Name,
       sum(case when Class = 1 then 1 else 0 end),
       sum(case when Class = 2 then 1 else 0 end),
       sum(case when Class = 3 then 1 else 0 end)
from myTable
group by Name

您可以将其旋转：

SELECT t.Name,
    SUM(if(t.Class=1,1,null)) as Class1,
    SUM(if(t.Class=2,1,null)) as Class2,
    SUM(if(t.Class=3,1,null)) as Class3
FROM myTable t
GROUP BY t.Name

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感谢您高效的回答，现在50000条条目的查询时间快多了。