CASE with aggregate functions in SQL - Step-by-Step Execution

Concept Flow - CASE with aggregate functions

Start Query

↓

Aggregate Function Calculates

↓

CASE Evaluates Condition on Aggregate Result

↓

WHEN condition True

→Return Value 1

↓

WHEN condition False

→Return Value 2

↓

Output Final Result

↓

End

The query first calculates an aggregate value, then the CASE statement checks conditions on that value to decide what output to return.

Execution Sample

SQL

SELECT
  department_id,
  CASE
    WHEN AVG(salary) > 5000 THEN 'High'
    ELSE 'Low'
  END AS salary_level
FROM employees
GROUP BY department_id;

This query groups employees by department, calculates average salary per department, and labels it 'High' or 'Low' based on the average.

Execution Table

Step	department_id	AVG(salary)	CASE Condition (AVG(salary) > 5000)	CASE Result	Output Row
1	10	6000	6000 > 5000 = True	'High'	(10, 'High')
2	20	4500	4500 > 5000 = False	'Low'	(20, 'Low')
3	30	5200	5200 > 5000 = True	'High'	(30, 'High')
4	40	3000	3000 > 5000 = False	'Low'	(40, 'Low')
5	End				Query complete, all groups processed

💡 All department groups processed, query ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
department_id	N/A	10	20	30	40	N/A
AVG(salary)	N/A	6000	4500	5200	3000	N/A
CASE Result	N/A	'High'	'Low'	'High'	'Low'	N/A

Key Moments - 2 Insights

Why does the CASE statement use the aggregate AVG(salary) instead of individual salaries?

Because the query groups rows by department_id, the aggregate AVG(salary) calculates one average per group. The CASE evaluates this single value per group, not individual salaries. See execution_table rows 1-4.

What happens if AVG(salary) equals exactly 5000?

The CASE condition is AVG(salary) > 5000, so if AVG(salary) is 5000, the condition is False and the ELSE branch 'Low' is returned. This is shown by the strict greater-than operator in the CASE condition.

Visual Quiz - 3 Questions

Look at the execution_table, what is the CASE Result for department_id 20?

A 'Low'

B 'High'

C NULL

D 5000

💡 Hint

Check the row where department_id is 20 in the execution_table under CASE Result column.

At which step does the AVG(salary) first exceed 5000?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the AVG(salary) values in execution_table rows and find the first greater than 5000.

If the CASE condition changed to AVG(salary) >= 5000, what would be the CASE Result for department_id 30?

A 'Low'

B 'High'

C NULL

D Error

💡 Hint

Refer to variable_tracker for AVG(salary) for department_id 30 and consider the new condition.

Concept Snapshot

CASE with aggregate functions:
- Aggregate functions (e.g., AVG, SUM) compute values per group.
- CASE evaluates conditions on these aggregate results.
- Syntax: CASE WHEN aggregate_condition THEN result ELSE other_result END
- Use GROUP BY to group rows before aggregation.
- Output depends on aggregate values per group.

Full Transcript

This visual execution shows how a SQL query uses CASE with aggregate functions. The query groups employees by department_id and calculates the average salary per group. Then, the CASE statement checks if the average salary is greater than 5000. If true, it outputs 'High'; otherwise, 'Low'. The execution table traces each department's average salary, the CASE condition evaluation, and the final output row. The variable tracker shows how department_id, average salary, and CASE results change step-by-step. Key moments clarify why CASE uses aggregate results and what happens at boundary values. The quiz tests understanding of CASE results and condition changes. This helps beginners see how CASE works with aggregates in SQL.