MySQL Integer column, unique values but default possibility?

I want a table with an integer column, that may or may not be filled (it is a social security number). But if it is filled, I want it to be UNIQUE : there cannot be two entries of the same number.

Using a unique constraint won't work cause integer won't accept NULL values, and MySQL detects multiple 0 values.

How can I set a unique constraint on 开发者_Go百科an integer with a default value ? Or how can I set the integer column to accept NULL values ? (this question takes it for granted : MySQL Foreign Key Constraint - Integer Column but I can't)

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create table test (
    myint INT NULL, UNIQUE INDEX (myint)
);

This will allow a unique constraint on any integers added but will allow multiple NULL values to be entered.

MySQL treats NULL as 'unknown' value so cant possibly do a comparison to see if a like value is already there 'unknown' !== 'unknown'.

This also depends on which database engine you are using, the above holds true for MyISAM and InnoDB

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Please see the code below:

ALTER TABLE test MODIFY myint INT NULL
ALTER TABLE test ADD UNIQUE INDEX (myint)

It works when data inputs are directly from MySQL (PHPMyadmin), saved as NULL, but a php script saves it as zeroes, and so it does not allow multiple entries.