Question

I have a table

students[std_id, name, class, gender,etc]

select class,gender,count(*) as total_students 
from students 
group by class,gender

its output is as below

 1st | male   | 23   
 1st | female | 11   
 2nd | male   | 17   

 2nd | female | 0   

//the last row is not shown because 0 female students in 2nd class

how to make it shown as above with total_sudents=0 instead of skipping record.

Answer 1

Use this solution:

SELECT    a.class,
          a.gender,
          COUNT(b.class) AS total_students
FROM      (
          SELECT     a.class, 
                     b.gender
          FROM       students a
          CROSS JOIN (
                     SELECT 'male' AS gender UNION ALL 
                     SELECT 'female'
                     ) b
          GROUP BY   a.class, 
                     b.gender
          ) a
LEFT JOIN students b ON a.class = b.class AND 
                        a.gender = b.gender
GROUP BY  a.class,
          a.gender