sql - mysql SELECT COUNT(*) ... GROUP BY ...不返回计数为零的行

Question

SELECT student_id, section, count( * ) as total
FROM raw_data r
WHERE response = 1
GROUP BY student_id, section

测试分为 4 个部分，每个部分都有不同数量的问题。我想知道，对于每个学生和每个部分，他们正确回答了多少个问题 (response=1)。

但是，对于此查询，如果学生在给定部分没有正确回答问题，则该行将完全从我的结果集中丢失。我怎样才能确保对于每个学生，总是返回 4 行，即使一行的“总计”为 0？

这是我的结果集:

student_id  section     total
1           DAP--29     3
1           MEA--16     2
1           NNR--13     1  --> missing the 4th section for student #1
2           DAP--29     1
2           MEA--16     4
2           NNR--13     2  --> missing the 4th section for student #2
3           DAP--29     2
3           MEA--16     3
3           NNR--13     3 --> missing the 4th section for student #3
4           DAP--29     5
4           DAP--30     1
4           MEA--16     1
4           NNR--13     2 --> here, all 4 sections show up because student 4 got at least one question right in each section

感谢您的任何见解!

更新:我试过了

 SELECT student_id, section, if(count( * ) is null, 0, count( * ))  as total

这根本没有改变结果。其他想法？

更新 2:由于以下回复，我让它工作了:

 SELECT student_id, section, SUM(CASE WHEN response = '1' THEN 1 ELSE 0 END ) AS total
 FROM raw_data r
 WHERE response = 1
 GROUP BY student_id, section

Answer 1

SELECT student_id, section, sum(case when response=1 then 1 else 0 end) as total
FROM raw_data_r GROUP BY student_id, section

请注意，没有 WHERE 条件。