How can I make an existing column have a unique constraint

1) Make sure that the column only conains unique values
lets assume we are working with a table called t5. it has two columns. id ( the primary key ) and name. To find out if the column contains any duplicate values make the following statements.

select name from t5 ;
select distinct name from t5 ;

IF the row counts are the same then the column contains unique values only.

2) Create a unique index as follows..

alter table t5 add unique ( name ) ;

If you want to give the index a name do it this way...

alter table t5 add unique namedex ( name )

3) Drink a beer

That's it!

* You may optionally drink beer between and during steps one and two.