php - 使用仅用于计数的 where 子句获取不同行及其记录计数

Question

-------------------------------------
Name of Student       |      class   
-------------------------------------
S1                            First
S11                           First
S12                           First
S13                           First
S2                            Second
S3                            Third
S31                           Third
-------------------------------------

现在我通过以下查询获得了不同的类及其每个计数

SELECT class,count(*) FROM students GROUP BY class

我明白了，一切都很好

array(
       "class"=> "First", "count"=>"4",
       "class"=> "Second", "count"=>"1",
       "class"=> "Third", "count"=>"2",
    ) 

但我只想将 where 条件应用于计数，因此我想要所有不同的类，并根据 where 子句进行计数

例如，如果我的条件是 class="First"

SELECT class,count(*) FROM students where class="First" GROUP BY class

该查询给了我这个

array(
       "class"=> "First", "count"=>"4"
    )

但我想要所有不同的类，所以我的预期结果如下

array(
       "class"=> "First", "count"=>"4",
       "class"=> "Second", "count"=>"0",
       "class"=> "Third", "count"=>"0",
    )  

感谢任何帮助，谢谢

Answer 1

使用SUM()计算符合所需条件的行数。

SELECT class, SUM(class = "First") AS count
FROM students
GROUP BY class