SQL Group By和Count函数

Question

我有以下数据库表，我想按颜色计算和分组。我是一年级学生，也是SQL的初学者。任何人都可以使用代码进行教学吗？

SQL-CountColor：

我尝试过：

Select COLOR,
  sum(case when Blue = 1 then 1 else 0 end) as Blue_count,
  sum(case when Red then 1 else 0 end) as Red_count,
  sum(case when Yellow then 1 else 0 end) as Yellow_count,
  sum(case when Black then 1 else 0 end) as Black_count,
  sum(case when Green then 1 else 0 end) as Green_count,
from TAB_GROUP
group by COLOR;

Answer 1

你应该搜索一下，这是一个非常常见的SQL语句。

select COLOR, count(*) from TAB_GROUP group by COLOR

Answer 2

select     sum(case when color = 'blue' then 1 else 0 end) as 'Blue',
           sum(case when color = 'red' then 1 else 0 end) as 'Red',
           sum(case when color = 'yellow' then 1 else 0 end) as 'Yellow',
           sum(case when color = 'Black' then 1 else 0 end) as 'Black',
           sum(case when color = 'Green' then 1 else 0 end) as 'Green'
From Table

Answer 3

您的查询基本正确。您只需删除GROUP BY并修复case即可引用数据中的列：

select sum(case when color = 'Blue' then 1 else 0 end) as Blue_count,
       sum(case when color = 'Red' then 1 else 0 end) as Red_count,
       sum(case when color = 'Yellow' then 1 else 0 end) as Yellow_count,
       sum(case when color = 'Black' then 1 else 0 end) as Black_count,
       sum(case when color = 'Green' then 1 else 0 end) as Green_count
from TAB_GROUP;

Answer 4

您的查询存在的问题是您正在混合两种方法，这两种方法都有效但不兼容。

第一个是使用case语句，就像@LONG在答案中所做的那样，它很好，但并不需要group by;你已经＆＃34;人为地＆＃34;通过在每列中给出不同的条件进行分组;

select  sum(case when Blue = 1 then 1 else 0 end) as Blue_count,
        sum(case when Red then 1 else 0 end) as Red_count,
        sum(case when Yellow then 1 else 0 end) as Yellow_count,
        sum(case when Black then 1 else 0 end) as Black_count,
        sum(case when Green then 1 else 0 end) as Green_count
from    TAB_GROUP

另一种方法是使用group by，它也很好，但你只需计算每组的行数

select  COLOR, count(*) as CNT
from    TAB_GROUP
group by COLOR

这将为您提供与所需结果相同但行和列反转的结果

COLOR  | CNT 
Blue   | 2
Red    | 2
Yellow | 1
Black  | 1
Green  | 1

要将行移动到列，您需要一个旋转功能，其语法可能会因您使用的数据库而异。这使得这种方法更加复杂，但在可能的数量增加的情况下也更加通用。