SQL中的三个别名

Question

我很困惑在SQL中使用AS查看三个条件，这是我的表

id | Fruit
___________
1  | Apel
2  | Orange
3  | Apel
4  | Manggo
___________

这是我的第一个查询：

SELECT
COUNT(fruit) AS apelfruit
FROM fruit_table
WHERE fruit = 'Apel'

输出：

apelfruit
_________
    2

但是当我这样做时：

SELECT
COUNT(fruit) AS apelfruit,
COUNT(fruit) AS orangefruit,
COUNT(fruit) AS manggofruit
FROM fruit_table
WHERE fruit = 'Apel'
AND fruit = 'Orange'
AND fruit = 'Manggo'

输出为零（0）：

 apelfruit | orangefruit | manggofruit
 ____________________________________
       0            0            0

我有点困惑什么是错的。任何人都可以帮助我，我非常感激。谢谢。

Answer 1

尝试以下方式

SELECT
sum(case when fruit = 'Apel' then 1 else 0 end) AS apelfruit,
sum(case when fruit = 'Orange' then 1 else 0 end) AS orangfruit,
sum(case when fruit = 'Manggo' then 1 else 0 end) AS manggofruit
FROM fruit_table

Answer 2

为了简洁起见，我更喜欢在这里使用COUNT：

SELECT
    COUNT(CASE WHEN fruit = 'Apel'   THEN 1 END) AS apelfruit,
    COUNT(CASE WHEN fruit = 'Orange' THEN 1 END) AS orangfruit,
    COUNT(CASE WHEN fruit = 'Manggo' THEN 1 END) AS manggofruit
FROM fruit_table;

正如@nicomp所指出的那样，以这种方式进行数据透视查询的限制是输出中出现的列是硬编码的，并且必须为新的条件计数添加更多代码。但是，唯一真正的方法是使用动态SQL。

Answer 3

条件是你的问题，水果不能等于“apel”和“芒果”它应该是“apel”或“apel”

由于只计算水果，将始终统计所有水果，你还需要计算一个条件，在这个解决方案中，总和1或0，基于条件：

SELECT
SUM(IF(fruit='Apel', 1,0)) AS apelfruit,
SUM(IF(fruit='Orange', 1,0)) AS orangfruit,
SUM(IF(fruit='Manggo', 1,0)) AS manggofruit
FROM fruit_table WHERE fruit IN ('Apel', 'Orange', 'Manggo')
;