Question

I am having trouble writing a query that will select all Skills, joining the Employee and Competency records, but only return one skill per employee, their newest Skill. Using this sample dataset

Skills
======
id   employee_id   competency_id   created
1    1             1               Jan 1
2    2             2               Jan 1
3    1             2               Jan 3

Employees
===========
id   first_name   last_name
1    Mike         Jones
2    Steve        Smith


Competencies
============
id   title
1    Problem Solving
2    Compassion

I would like to retrieve the following data

Skill.id   Skill.employee_id   Skill.competency_id   Skill.created   Employee.id   Employee.first_name   Employee.last_name   Competency.id   Competency.title
2          2                   2                     Jan 1           2             Steve                 Smith                2               Compassion
3          1                   2                     Jan 3           1             Mike                  Jones                2               Compassion

I was able to select the employee_id and max created using

SELECT MAX(created) as created, employee_id  FROM skills GROUP BY employee_id

But when I start to add more fields in the select statement or add in a join I get the ‘Column ‘xyz’ is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.’ error.

Any help is appreciated and I don’t have to use GROUP BY, it’s just what I’m familiar with.

Answer 1

The error that you were getting is because SQL Server requires any item in the SELECT list to be included in the GROUP BY if there is an aggregate function being used.

The problem with that is you might have unique values in some columns which can throw off the result. So you will want to rewrite the query to use one of the following:

You can use a subquery to get this result. This gets the max(created) in a subquery and then you use that result to get the correct employee record:

select s.id SkillId,
  s.employee_id,
  s.competency_id,
  s.created,
  e.id employee,
  e.first_name,
  e.last_name,
  c.id competency,
  c.title
from Employees e
left join Skills s
  on e.id = s.employee_id
inner join
(
  SELECT MAX(created) as created, employee_id  
  FROM skills 
  GROUP BY employee_id
) s1
  on s.employee_id = s1.employee_id
  and s.created = s1.created
left join Competencies c
  on s.competency_id  = c.id

See SQL Fiddle with Demo

Or another way to do this is to use row_number():

select *
from
(
  select s.id SkillId,
    s.employee_id,
    s.competency_id,
    s.created,
    e.id employee,
    e.first_name,
    e.last_name,
    c.id competency,
    c.title,
    row_number() over(partition by s.employee_id 
                      order by s.created desc) rn
  from Employees e
  left join Skills s
    on e.id = s.employee_id
  left join Competencies c
    on s.competency_id  = c.id
) src
where rn = 1

See SQL Fiddle with Demo