how NULL workspic.twitter.com/0l3nYbogTB
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something i want to think through a bit more is that NULL actually has at least 2 different possible meanings: 1. the data is missing ("this person has a name but it's not in our database") 2. the data doesn't exist ("this cat has no owner")
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For a lot of folks who learned with a system that discouraged using foreign keys, it may also mean "this cat points to an invalid owner"
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i'm not sure what you mean -- what's the difference between "an invalid owner" and "doesn't have an owner" here?
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`owner_id = NULL` has semantic value. `owner_id = 1` where no owner record had the ID 1 is a clearly invalid state that shouldn't ever occur.
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ON DELETE SET NULL
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