How to use ALTER TABLE to add a new column and make it unique?

Question

How do I use ALTER TABLE to add a new column and make it unique?

Answer 1

Depends on the DBMS, but I think the following is quite portable:

ALTER TABLE table_name ADD column_name datatype
ALTER TABLE table_name ADD UNIQUE (column_name)

If you want to give a name to the UNIQUE constraint, you could replace the last command with this:

ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name)

Answer 2

if table is empty

  ALTER TABLE ADD (FieldName Type)
  ALTER TABLE ADD CONSTRAINT UNIQUE(FieldName)

If you have data in table you need to this in three steps:

1. Add column
2. Fill values
3. Add unique constraint

Answer 3

It is a two step process: add the new coloumn, then add the constraint. Because UNIQUE constraints permit nulls it doesn't matter whether the table is populated:

SQL> select count(*) from t23
  2  /

  COUNT(*)
----------
         2


SQL> alter table t23
  2      add new_col number
  3  /

Table altered.

SQL> alter table t23
  2      add constraint t23_uk unique (new_col)
  3  /

Table altered.

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