UPDATE based on value in another table

[Dan Jones]

How do I update a table to remove orphaned references to a second
table? I've deleted rows in the second table, which has a unique
auto_increment key. The first table now has references to keys that no
longer exist. I need to update the first table, setting the value to
NULL where the referenced key no longer exists. Something like:

UPDATE table1 SET table2ID = NULL WHERE table1.table2ID NOT FOUND IN
table2.ID;

The NOT FOUND IN isn't SQL, of course, but I'm not sure what should go
there.
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