Is there any way we can exclude all possible values ?

hi all,

   Is there any way we can exclude all possible values to be assigned to a column in a table without knowing the type of the column ?    For example,suppose the name of the column is COL,if the type of the column is number,perhaps we want to return COL =a, since the type of the column is number,so the value to be assigned to it can not be 'a',through this we can exclude all the possible value.    Though this seems to be queer, but in our project, we have this need.    

   Can anyone tell my how to do?

   Thank you in advance
   Best Regards    

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