NULL and Unique Indexes in Teradata

For unique indexes, Teradata Database treats nulls as if they are equal rather than unknown.
For single-column unique indexes, only one row may have null for the index value; otherwise uniqueness violation error occurs.
For multi-column unique indexes, no two rows can have nulls in the same columns of the index and also have non-null values that are equal in the other columns of the index.

For example, consider a two-column index. Rows can occur with the following index values:
An attempt to insert a row that matches any of these rows will result in a uniqueness violation.

(Value of First Column in Index)      (Value of Second Column in Index)
10                          null
null                          10
null                                            null