How sum with case conditional statement works in sql

Question

The other day, I gave an answer to this question but then other user solved that problem with sum + case conditional statement to add one edge condition in result. So, the q

Answer 1

Presumably, this is the part that you are struggling to understand:

  select deptno,
         sum(case when jobname = 'Analyst' then 1 else 0 end) as numAnalysts
  from employees
  group by deptno

This is a simple aggregation query, really. What the query is doing is:

• Look at each row in employees
• If jobname is 'Analyst' then assign the value of 1 (this is the case statement. Otherwise, assign a value of0`.
• Aggregate by department, summing the value just calculated. This has the effect of counting the number of analysts.

case is an expression that returns a value. The sum() is simply adding up that value for each group.