如何在SQL查询中为筛选列创建别名

Question

下面的SQL查询给出了我想要的结果，但使用CASE语句似乎不正确。 是否有另一种方法为过滤列创建别名？有没有更有效的方法来实现相同的结果？

以下是当前查询：

SELECT 
      year, 
      college_name, 
      count(id_999999999) as count_id_999999999,
      count(id_999999999)/count(student_id) as percent_id_999999999,
      count(id_other) as count_id_other,
      count(id_other)/count(student_id) as percent_id_other,
      count(student_id) as total_id

FROM      (SELECT fiscal_year, semester, 

                 CASE
                     WHEN student_id IN ('999999999') THEN 'id - 999999999' 
                 END
                 AS id_999999999,

                 CASE
                     WHEN student_id NOT IN ('999999999') THEN 'id - Other'
                 END
                 AS id_other,

                student_id

            FROM enrolment_data) enrol

GROUP BY    year, college_name
ORDER BY    year, college_name;

以下是理想的结果：

Answer 1

查询过于复杂。你不需要子查询。我会把查询写成：

SELECT year, college_name, 
       sum(case when student_id IN ('999999999') then 1 else 0 end) as count_id_999999999,
       avg(case when student_id IN ('999999999') then 1.0 else 0 end) as percent_id_999999999,
       sum(case when student_id NOT IN ('999999999') then 1 else 0 end) as count_id_other,
       avg(case when student_id NOT IN ('999999999') then 1.0 else 0 end) as percent_id_other
       count(*) as total_id
FROM  enrolment_data ed
GROUP BY year, college_name
ORDER BY year, college_name;