SQL - Making 4 new columns in a result from another column - sql

Question

So, I'm making a data base for my college class, it's about a foreign languages school, and I need to ( using a single query ), have a number of people that are attending a certain language class, but it has to be seperated by the age group. For example, this is how the result table should look like:
Language | 14-25 | 25-35 | 35-50 | 50+ |
German | 1 | 0 | 0 | 0 |
Italian | 2 | 1 | 0 | 0 |
English | 5 | 0 | 0 | 0 |
I need to do this by joining the tables "Class" that has attributes (Language, Number of students), and "Student" that has attributes (ID, name, surname, age, prior knowledge ( eg. A1, B2, ... ))
So I somehow have to figure out in which age group a certain individual goes to, then if he goes there, increment the number of students for that age group by one.

Answer 1

You can build the sum and group the entries using CASE WHEN, so your query will look like this:
SELECT c.language,
SUM(CASE WHEN s.age BETWEEN 14 AND 25 THEN 1 ELSE 0 END) AS '14-25',
SUM(CASE WHEN s.age BETWEEN 25 AND 35 THEN 1 ELSE 0 END) AS '25-35',
SUM(CASE WHEN s.age BETWEEN 35 AND 50 THEN 1 ELSE 0 END) AS '35-50',
SUM(CASE WHEN s.age >= 50 THEN 1 ELSE 0 END) AS '50+'
FROM class c
JOIN student_class sc ON c.language = sc.class_language
JOIN student s ON s.id = sc.student_id
GROUP BY c.language;
You have to take care because as example a person whose age is 25 will be selected in both groups "15-25" and "25-35". If this is not intended, you could do something like this:
...SUM(CASE WHEN s.age BETWEEN 14 AND 25 THEN 1 ELSE 0 END) AS '14-25',
SUM(CASE WHEN s.age BETWEEN 26 AND 35 THEN 1 ELSE 0 END) AS '25-35',
SUM(CASE WHEN s.age BETWEEN 36 AND 50 THEN 1 ELSE 0 END) AS '35-50',
SUM(CASE WHEN s.age > 50 THEN 1 ELSE 0 END) AS '50+'...
Please see the working example here: db<>fiddle
You could add an ORDER BY c.language at the end if you want.
A last note: The column aliases shown here ('14-25' etc.) will not work on every DB type and might be replaced depending on DB type and personal "taste".

Answer 2

Assuming you have a table called something like ClassStudent which is linking the individual students to the class (which you absolutely need to fulfil this requirement)...
SELECT c.Language,
[14-25] = SUM(IIF(s.age BETWEEN 14 AND 25, 1, 0)),
[25-35] = SUM(IIF(s.age BETWEEN 25 AND 35, 1, 0)),
[35-50] = SUM(IIF(s.age BETWEEN 35 AND 50, 1, 0)),
[50+] = SUM(IIF(s.age >= 50, 1, 0)),
FROM Class c
INNER JOIN ClassStudent cs ON c.Language = cs.Language /* you need this table */
INNER JOIN Student s ON cs.StudentID = s.ID
GROUP BY c.Language
Here, IIF is like a ternary operator in SQL form, and the SUM lets you count up where the condition is met.