SQL Query to change group clause - mysql

Question

I have a query as follows:
SELECT age_groups.Name as name, avg(scores.score) as avg
FROM scores
JOIN users ON scores.id = users.id
JOIN age_groups on user.age_group = age_groups.id;
There are 5 age groups: 0-10, 11-20, 21-30, 31-40, 41-50.
I'd like to have only three age groups in my results: 0-30, 31-40, and 41-50.
What statements would allow me to group three age groups together?
EDIT:
The age_groups table looks like this:
ID - Name
1 - 0-10
2 - 11-20
3 - 21-30
...etc

Answer 1

You can use case to combine age groups. First, though, your query is missing group by, so it should read more like:
SELECT ag.Name as name, avg(s.score) as avg
FROM scores s JOIN
users u
ON s.id = u.id JOIN
age_groups ag
on u.age_group = ag.id
GROUP BY ag.Name;
You can then do what you want as:
SELECT (case when ag.Name in ('0-10', '11-20', '21-30') then '0-30'
else ag.Name
end) as MyAgeGroup, avg(s.score) as avg
FROM scores s JOIN
users u
ON s.id = u.id JOIN
age_groups ag
on u.age_group = ag.id
GROUP BY (case when ag.Name in ('0-10', '11-20', '21-30') then '0-30'
else ag.Name
end);

Answer 2

One solution is to replace "age_groups.Name" in your select clause with a case statement which remaps each of the contributing ranges to your desired range:
CASE age_groups.Name
WHEN '0-10'
THEN '0-30'
WHEN '10-20'
THEN '0-30'
WHEN '20-30'
THEN '0-30'
ELSE age_groups.Name
END AS Name
and also add this to the bottom with a GROUP BY clause (e.g., GROUP BY CASE WHEN ...)
However, if this type of thing might be done frequently, then I would recommend a longer term solution, which is to add numeric bounds to the age groups in your age group table such as Age_Group_Lower_Bound (integer) and Age_Group_Upper_Bound (integer) (e.g., your 0-10 row would then have Age_Group_Lower_Bound=0 and Age_Group_Upper_Bound=10). Once these are in place, your original query could use these, e.g.,:
CASE
WHEN age_groups.Age_Group_Upper_Bound <= 30
THEN '0-30'
ELSE age_Groups.Name
END
Here's an example of that, taking it further with more ranges getting grouped together, e.g.:
CASE
WHEN age_groups.Age_Group_Upper_Bound <= 20
THEN '0-20'
WHEN age_groups.Age_Group_Upper_Bound <= 40
THEN '20-40'
WHEN age_groups.Age_Group_Upper_Bound <= 60
THEN '40-60'
ELSE '60+'
END