Transposing mysql rows into customized column with data

Question

I have following table in mysql

emp_id          month           salary
1               Jan             2000
1               Feb             2000
1               March           2000
2               Jan             3000
2               Feb             3000
2               March           3000

I am trying to get a result like following:

emp_id   Jan_sal    feb_sal     Mar_sal
1           2000    2000        2000
2           3000    3000        3000

My query:

select emp_id,
(case when month = 'Jan' then salary else NUll end ) jan_sal,
(case when month = 'Feb' then salary else NUll end ) feb_sal,
(case when month = 'March' then salary else NUll end ) march_sal
from emp group by emp_id;

But the output I get as:

emp_id jan_sal  feb_sal march_sal
1      2000     NULL    NULL
2      3000     NULL    NULL

Not sure if my query is right. Any help is appreciated.

Answer 1

You want to use an aggregate function like SUM or MAX on the case statements

select emp_id,
sum(case when month = 'Jan' then salary else NUll end ) jan_sal,
sum(case when month = 'Feb' then salary else NUll end ) feb_sal,
sum(case when month = 'March' then salary else NUll end ) march_sal
from emp
group by emp_id;

If multiple salaries are present (may be other benefits etc) for same employee, then you have to decide if you want to add all the salaries or take only the max value and use SUM or MAX based on that.

Answer 2

Use the MAX() function to pivot:

SELECT emp_id,
       MAX(CASE WHEN month = 'Jan'   THEN salary END) AS jan_sal,
       MAX(CASE WHEN month = 'Feb'   THEN salary END) AS feb_sal,
       MAX(CASE WHEN month = 'March' THEN salary END) AS march_sal
FROM emp
GROUP BY emp_id;

This assumes that each employee only has one salary entry per month. If there could be more than one entry, then use should use your original CASE expressions with SUM() instead of MAX() .