Storing empty strings vs NULLs in MySQL database?

I have a PHP/MySQL application that stores "开发者_开发技巧blank" values in some cases as '' (empty strings) and in other cases as NULLs.

Having this mixed format certainly causes a problem when comparing, so I’m wondering which one is the better storage mechanism? '' or NULL?

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While this is confusing, actually you should store null for a number of reasons :

1. Checking against null is usually faster than checking an empty string in most databases
2. Null commonly means "i don't know", empty string means "I know : it's empty". It gives you better semantics.