Understanding SQL NULL

NULL means 'unknown' or 'missing' — not zero, not an empty string, and genuinely not equal to anything, including another NULL.

What & Why

NULL represents an absent or unknown value. A missing product.feedback.nps_score does not mean a score of zero or an empty string; it means no NPS score was submitted for that feedback row.

NULL is not equal to anything, including another NULL. NULL = NULL evaluates to UNKNOWN, which is why SQL provides IS NULL and IS NOT NULL for missing-value tests.

See How It Works

BUSINESS QUESTION

Product wants to compare = NULL with IS NULL against the real nullable nps_score column.

id	user_id	feature_id	rating	nps_score	comment	created_at
1	101	1	5	9	Clear and useful	2024-01-25 12:15:00+00
2	102	2	4	NULL	Export worked well	2024-02-20 15:40:00+00
3	103	1	4	7	Helpful setup flow	2024-03-18 09:05:00+00
4	104	3	3	NULL	Needs clearer timing	2024-04-08 17:30:00+00

Watch SQL distinguish comparison from IS NULLStep 1 of 4

nps_score = NULL

id	nps_score	expression	result
2	NULL	NULL = NULL	UNKNOWN

Ordinary equality with NULL is UNKNOWN—even when the stored NPS score is NULL.

EXAMPLE QUERY

SELECT
  id,
  nps_score,
  nps_score = NULL AS equals_null,
  nps_score IS NULL AS is_null
FROM product.feedback
ORDER BY id;

Now You Try

Practice this concept

Product compares = NULL with IS NULL against the real nullable nps_score column.

Available schema

product

Prefix tables with product.table_name.

idnps_scoreequals_nullis_null

product.feedback

Column	Type
id	integer
user_id	integer
feature_id	integer
rating	integer
nps_score	integer
comment	text
created_at	timestamp with time zone
moderation_bucket	text

query.sql

Intermediate business practice