The comments in the qa-external-users.php file should explain everything.
In your users table (the external one) you should have a unique way of identifying a user - most likely an ID number, but you could also just use the username or email address.
In MySQL that field will have a type. The function qa_get_mysql_user_column_type() should return what that type is. You can return any of these (inside a string):
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SMALLINT
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SMALLINT UNSIGNED
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MEDIUMINT
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MEDIUMINT UNSIGNED
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INT
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INT UNSIGNED
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BIGINT
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BIGINT UNSIGNED
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VARCHAR(x) where x is the maximum length - for example, VARCHAR(30)