如何对两行SQL查询求和

Question

In a database I have registrations vof students for an event. It looks like this:

id, firstname, surname, gender, email, studydegree, study, workshop1, workshop2, participate_speeddating, ss_companies, date of birth, date of registration

for example

6574, John, Doe, Male, johndoe@gmail.com, Phd, ComputerScience, None, None, no, None, 01-01-1990, 01-01-2017

I have the following the query:

    <?php query_to_html('Studies', 'SELECT study, COUNT(*) FROM registrants GROUP BY study'); ?> 

where query_to_html transforms the query into an html table. This gives me the following:

study , COUNT(*)
Mathematics, 90
Physics, 71
Astronomy, 5
Biology, 25
Biomedical Sciences, 25
... etc

I'd like to sum some of the studies, based on study association (array), for example group 1) Mathematics, Astronomy, Physics and Computer Science and group 2) Biology and Biomedical sciences. Like this:

study_association, amount
Fundamentals, 166
Bio, 50

where 166 and 55 is a sum of a few rows corresponding to the study_association array.

How do I do this?

Thanks!

Answer 1

You can use SUM(IF()) to count only specific rows. I assume your column is "group":

SELECT
    study,
    COUNT(*) AS sum,
    SUM(IF(`group` IN ('Mathematics', 'Astronomy', 'Physics', 'Computer Science'), 1, 0) AS group1,
    SUM(IF(`group` IN ('Biology', 'Biomedical'), 1, 0) AS group2
FROM registrants
GROUP BY study

This will count registrants which are in one of group in given array.