I have an existing table with data. I have just added a new column but I cannot find how to add a unique constraint on that column. Could someone please advise? Right-clicki
Do right-click in the table designer and choose Indexes/Keys.

Indexes/Keys window will open. Click the button Add, to create the new index/key, and choose the column to be unique in the Columns property:

In the properties of the new index/key, set Type to Unique Key:

And this is the generated code for the unique constraint:
ALTER TABLE [dbo].[Table_1] ADD CONSTRAINT [IX_Table_1] UNIQUE NONCLUSTERED
(
[myUniqueColumn] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO