SQL COALESCE (NULL Handling)

Returns the first non-NULL value from a list — the standard tool for substituting a fallback value wherever NULL shows up.

What & Why

COALESCE(a, b, c, ...) evaluates each argument in order and returns the first one that isn't NULL. With two arguments — COALESCE(column, fallback) — this reads as "use the column's value, or this fallback if it's NULL."

See How It Works

BUSINESS QUESTION

Product wants feedback comments to show a clear fallback when no comment exists.

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,
  COALESCE(NULLIF(TRIM(comment), ''), 'No comment') AS feedback_text
FROM product.feedback
ORDER BY feedback_id;

RESULT — exact output from the displayed Queryflo rows

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

All displayed comments are present, so COALESCE returns their stored text.

Now You Try

Practice this concept

Product wants feedback comments to show a clear fallback when no comment exists.

Available schema

product

Prefix tables with product.table_name.

feedback_idfeedback_text

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