The last half an hour has confirmed to me that I know so little when it comes to MySQL. I have been trying to update the value of a column (guest_account) in a table (customers) where guest_account = NULL. The updated value should be 0. In other words, in the customers table, change the value of all entries in the guest_account column/field from NULL to 0, but only if they equal NULL.
Here's what I thought would work (but doesn't):
UPDATE `customers` SET `guest_account` = '0' WHERE `guest_account` = NULL;