HAVING for filtering groups in PostgreSQL - Step-by-Step Execution

Concept Flow - HAVING for filtering groups

Start with Table Data

↓

GROUP BY specified columns

↓

Calculate aggregates per group

↓

Apply HAVING condition on groups

Yes | No↓

Exclude group

↓

Return groups that meet HAVING condition

Data is grouped by columns, aggregates are calculated, then HAVING filters groups based on conditions.

Execution Sample

PostgreSQL

SELECT department, COUNT(*) AS emp_count
FROM employees
GROUP BY department
HAVING COUNT(*) > 2;

This query groups employees by department and returns only departments with more than 2 employees.

Execution Table

Step	Action	Groups Formed	Aggregate Calculated	HAVING Condition	Group Included?
1	Group employees by department	[Sales, HR, IT]	Count employees in each group	N/A	N/A
2	Calculate counts	[Sales=3, HR=2, IT=4]	Sales=3, HR=2, IT=4	N/A	N/A
3	Apply HAVING COUNT(*) > 2	Same groups	Same counts	Sales: 3 > 2 = True	Yes
4	Apply HAVING COUNT(*) > 2	Same groups	Same counts	HR: 2 > 2 = False	No
5	Apply HAVING COUNT(*) > 2	Same groups	Same counts	IT: 4 > 2 = True	Yes
6	Return filtered groups	[Sales, IT]	Counts: 3, 4	Condition met	Included

💡 All groups checked; only those with count > 2 are included.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Groups	None	[Sales, HR, IT]	[Sales=3, HR=2, IT=4]	[Sales=3, HR=2, IT=4]	[Sales=3, IT=4]
HAVING Condition Result	N/A	N/A	N/A	[True, False, True]	[True, False]

Key Moments - 2 Insights

Why doesn't HAVING filter individual rows like WHERE?

HAVING filters after grouping and aggregation (see execution_table rows 3-5), so it works on groups, not single rows.

What happens if HAVING condition is false for a group?

That group is excluded from the final result (see execution_table row 4 where HR group is excluded).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the HAVING condition result for HR group?

A True

B Not evaluated

C False

D Error

💡 Hint

Check the 'HAVING Condition' column at step 4 for HR group.

At which step are groups formed from the table data?

A Step 2

B Step 1

C Step 3

D Step 6

💡 Hint

Look at the 'Action' column to find when grouping happens.

If HAVING condition changed to COUNT(*) > 4, which groups would be included?

A No groups

B Only IT

C Only Sales

D Sales and IT

💡 Hint

Refer to 'Aggregate Calculated' counts and compare with new condition.

Concept Snapshot

HAVING filters groups after GROUP BY and aggregation.
Syntax: SELECT columns FROM table GROUP BY columns HAVING condition;
HAVING works on aggregated data, unlike WHERE.
Use HAVING to filter groups, e.g., HAVING COUNT(*) > 2.
Groups failing HAVING condition are excluded from results.

Full Transcript

The HAVING clause filters groups formed by GROUP BY after aggregates are calculated. First, data is grouped by specified columns. Then, aggregate functions like COUNT calculate values per group. HAVING applies conditions on these aggregates to decide which groups to keep. Groups not meeting the HAVING condition are excluded from the final output. This differs from WHERE, which filters rows before grouping. For example, grouping employees by department and counting them, HAVING COUNT(*) > 2 returns only departments with more than two employees.