Solution:Step 1: Understand HAVING with NULL groupsHAVING filters groups after grouping; condition Category IS NULL should match NULL group.Step 2: Identify why zero rows returnedSome SQL engines treat NULL in HAVING differently; better to use COALESCE or check data carefully.Final Answer:HAVING fil

[Solved] A query:SELECT Category, COUNT(*) FROM Products GROUP BY Category HAVING Category IS NULL;returns zero rows, but you know some rows have NULL Category. What is the issue? | SQL

SQL - GROUP BY and HAVING

A query:
SELECT Category, COUNT(*) FROM Products GROUP BY Category HAVING Category IS NULL;
returns zero rows, but you know some rows have NULL Category. What is the issue?

AHAVING filters groups after grouping; NULL groups exist but condition is wrong.

BHAVING clause cannot compare NULL with = or IS NULL correctly.

CNULL groups are excluded by default in GROUP BY.

DCOUNT(*) excludes NULL rows.

Step-by-Step Solution

Solution:

Step 1: Understand HAVING with NULL groups
HAVING filters groups after grouping; condition Category IS NULL should match NULL group.
Step 2: Identify why zero rows returned
Some SQL engines treat NULL in HAVING differently; better to use COALESCE or check data carefully.
Final Answer:
HAVING filters groups after grouping; NULL groups exist but condition is wrong. -> Option A
Quick Check:
HAVING filters groups; NULL condition may behave unexpectedly [OK]

Quick Trick: HAVING filters groups; test NULL conditions carefully [OK]

Common Mistakes:

MISTAKES

Assuming NULL groups are excluded by GROUP BY
Misusing HAVING with NULL comparisons
Thinking COUNT(*) ignores NULL rows

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A query:

Step 1: Understand HAVING with NULL groups

Step 2: Identify why zero rows returned

Final Answer:

Quick Check:

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