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When editing a post from mysqlman, I click the Null checkbox for those fields I want to be left blank, but it's not saving my changes.
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You have to delete everything from the column when you select it as null otherwise it won't change.

eg...

Column_Name [ Existing Content ] [x]

If you just hit X to make it null (X is just my version of a tickbox :) ) then you will still see Existing Content in the column.....so you need to erase it first (or at the time of checking the tickbox) eg:

Column_Name [ ] [X]

...then it will become null.