SQL NULL vs Empty String

An empty string '' is a real, zero-length piece of text — a known value. NULL is the absence of any text value whatsoever. Postgres treats these as genuinely distinct.

What & Why

A text column holding '' (empty string) has an actual, known value — it's just zero characters long. A text column holding NULL has no value recorded at all. Postgres keeps these strictly separate: '' IS NULL evaluates to FALSE, and '' = NULL evaluates to NULL (unknown), never TRUE.

See How It Works

BUSINESS QUESTION

Product normalizes blank feedback comments while preserving meaningful comments.

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

EXAMPLE QUERY

SELECT
  id AS feedback_id,
  comment,
  NULLIF(TRIM(comment), '') AS normalized_comment
FROM product.feedback
ORDER BY feedback_id;

RESULT — exact output from the displayed Queryflo rows

feedback_id	comment	normalized_comment
1	Clear and useful	Clear and useful
2	Export worked well	Export worked well
3	Helpful setup flow	Helpful setup flow
4	Needs clearer timing	Needs clearer timing

Every displayed comment is meaningful, so NULLIF preserves each value.

Now You Try

Practice this concept

Product wants blank and whitespace-only feedback comments normalized to NULL.

Available schema

product

Prefix tables with product.table_name.

feedback_idcommentnormalized_comment

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