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NULL isn’t equal (or not equal!) to anything in SQL (x = NULL and x != NULL are never true for any x). This results in 2 behaviours that are surprising at first:
Surprise! x= NULL doesn’t work
fish
name: NULL owner: bob
name: nemo owner: ahmed
SELECT * FROM fish
WHERE name = NULL
no results!
You need to use x IS NULL instead.
works
name IS NULL
name IS NOT NULL
doesn’t work
name = NULL
name != NULL
surprise! name != ‘betty’ doesn’t match NULLs
fish
name: NULL owner: bob
name: nemo owner: ahmed
SELECT FROM fish
WHERE name != 'betty'
name: NULL owner: bob
To match NULLS as well, I’ll often write something like WHERE name = 'betty' OR name IS NULL instead.
more surprising truths
More operations with NULL which might be surprising:
2 + NULL => NULL
NULL * 10 => NULL
CONCAT(‘hi’, NULL) => NULL
NULL = NULL => NULL (NULL isn’t even equal to itself!)
2 = NULL => NULL
2 != NULL => NULL
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