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Editorial Team
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Asked: 2026-06-13T18:45:47+00:00

I’m trying to learn how to use CASE/WHEN in SQL more specifically, mySQL. I

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I’m trying to learn how to use CASE/WHEN in SQL more specifically, mySQL.

I have two tables: adhoc_item and adhoc_vulnerability. ahdoc_item has one adhoc_vulnerability related to it. now adhoc_vulnerability may or may not have a vulnerability (original_vulnerability_id) attached to it.

In case it doesn’t, then I want to return the name on itself.

The tables: (or relevant parts thereof)

ADHOC_ITEM
enter image description here

ADHOC_VULNERABILITY
enter image description here
This is my query:

SELECT adi.name as item, CASE ahv.original_vulnerability_id
WHEN ISNULL(ahv.original_vulnerability_id) THEN ahv.name
ELSE 'foo'
END NAME
FROM adhoc_item adi
JOIN adhoc_vulnerability ahv ON ahv.id = adi.adhoc_vulnerability_id

and these are the results!

enter image description here

Now I know this is a little bit complex but i’ve been thinking about it for about 2 hours and found no explanation on why that last item (which has an adhoc_vulnerability with NULL as original_vulnerability_id) shows the value that is supposed to appear precisely when that attribute is not null!

Thanks in advance.

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1 Answer

  1. Editorial Team

    The problem is that when ahv.original_vulnerability_id is NULL, the ISNULL() function returns TRUE (which is MySQL is the same as 1 but this is irrelevant). So, instead of comparing the column with NULL, you are comparing it with TRUE.

    Rewrite the CASE expresion as:

    CASE 
WHEN ISNULL(ahv.original_vulnerability_id) THEN ahv.name
ELSE 'foo'
END NAME

    or as:

    CASE 
WHEN ahv.original_vulnerability_id IS NULL THEN ahv.name
ELSE 'foo'
END NAME
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