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Hope the query below is fulfilled your requirement.

insert into output(select e.eno,e.ename,e.deptno from emp e,dept d where e.deptno=d.deptno)

Alter is a DDL command,you can not manipulate the table using alter. Need to use either update or insert into query to fill other column values.

You can have any number of constraints.There is no restriction in that.