SQL Server - Unique index vs Unique constraint - Re. Duplicate values - sql-server

Question

A unique index ensures that the values in the index key columns are unique.
A unique constraint guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. When a unique constraint is created a corresponding unique index is automatically created on the column(s).
Qusetions:
Can duplicate values be inserted if we have a unique index on a column and no unique constraint?
What about existing duplicates in any on the column - will it allow to create unique index or unique constraint?

Answer 1

Can duplicate values be inserted if we have a unique index on a column
and no unique constraint?
Generally, duplicate values cannot be inserted and an error is raised when a unique index exists on the column. The exceptions are:
Index was created with the IGNORE_DUP_KEY option. No error is raised and the insert is ignored.
The non-clustered index is filtered such that the duplicate value does not satisfy the index WHERE clause. The row is inserted but not reflected in the non-clustered index.
What about existing duplicates in any on the column - will it allow to
create unique index or unique constraint?
No, with the exception of the filtered index mentioned above.

Answer 2

Can duplicate values be inserted if we have a unique index on a column and no unique constraint?
No, the values of the columns within the index must create a unique set of data within that index.
What about existing duplicates in any on the column - will it allow to create unique index or unique constraint?
No, you cannot create a Unique Index on a table that has duplicate values.
This easiest way to have found this out would be to try (I suggest for things like that doing so, it's a great way of learning):
CREATE TABLE dbo.SomeTable (SomeInt int, AnotherInt int);
GO
INSERT INTO dbo.SomeTable (SomeInt,
AnotherInt)
VALUES (1,1),
(1,2),
(2,1);
GO
--Create a unique index on a column with duplicate values
CREATE UNIQUE INDEX UQ_SomeInt ON dbo.SomeTable(SomeInt); --fails
GO
--Create a unique index on the 2 columns, as they are unique
CREATE UNIQUE INDEX UQ_Some_AnotherInt ON dbo.SomeTable(SomeInt, AnotherInt); --Succeeds
GO
--Try to insert a duplicate value
INSERT INTO dbo.SomeTable (SomeInt,
AnotherInt)
VALUES(2,1); --fails
GO
SELECT *
FROM dbo.SomeTable
GO
DROP TABLE dbo.SomeTable;

Answer 3

One potentially unintuitive scenario that confused me at first: postgres does not treat NULL values as equal. If your table looked like this:
+-------+-------+-------+
|id |a |b |
+-------+-------+-------+
|1 |0 |NULL |
|2 |0 |NULL |
+-------+-------+-------+
You could still add a unique index on columns a and b. According to Postgres, row with id 1 and row with id 2 have the same value for column a, but different values for column b