mySQL计数表有效

Question

基本上我有一个表名myTable，里面有Name，Class，我想像下表那样对它们进行排序，每个名称的数量和1,2,3s的数量。下面是代码我目前有，但它可以处理少量条目，如果表格有20,000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;

Answer 1

这是经典的条件聚合：

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

Answer 2

使用条件计数执行GROUP BY：

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，它适用于整个选定的行。 （如果将列名放在括号内，则无关紧要。）

select distinct(col1), col2, ...

与

相同

select distinct col1, col2, ...

也与

相同

select distinct col1, (col2), ...

Answer 3

你可以转动它：

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