SQL 获取 id 1 列的平均值，按 id 2 同一列中的值分组

Question

The same column in my table needs to be used for Averaging and Grouping By. The catch is that the id is different when implementing each function.

My assignment questions are "What is your age" and "What is your gender".

I found the top 7 genders, these are: 'Male' , 'Female' , 'male' , 'female' , '-1' , 'Nonbinary' , 'non-binary' .

AnswerText contains the answers for each question. I want to get the Average age for each gender category in the list. I have done this:

SELECT AVG(AnswerText), Gender
FROM 
(
  SELECT AnswerText as Gender
  FROM Answer
  WHERE Answer.QuestionID = 2 
  AND Gender IN ('Male', 'Female', 'male', 'female', '-1', 'Nonbinary', 'non-binary')
)
WHERE Answer.QuestionID = 1
GROUP BY Gender

This is throwing error no such column: AnswerText .

How do I achieve this? The solution can either be in MySQL or SQLite.

Answer 1

I want to get the Average age for each gender category in the list.

That's a self-join:

select a2.answertext as gender, avg(a1.answertext * 1.0) as avg_age
from answer a1
inner join answer a2 on a2.userid = a1.userid
where a1.questionid = 1 and a2.questionid = 2
group by a2.answertext