Can unique keys be null?
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Create table emp (id number(9), name varchar2(20),salary number(9,2)); The table has 100 records after table created.Now i nee to change id's Datatype is to be Varchar2(15). now Alter table emp modify(id varchar2(15),name varchar2(20), salary number(9,2)); Whether it will work or returns error? post answer with explanation.
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