SQL Server 2022

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create table t (a varchar(20));
insert into t (a) values ('Hello-World'), ('Theremin'), ('-123');
3 rows affected

alter table t add clean_data as case when charindex('-', a) = 0 
                                then a
                                else substring(a, 0, charindex('-', a) ) 
                                end;
alter table t add constraint uq_clean_data unique (clean_data);
insert into t (a) values ('Happy');
1 rows affected

insert into t (a) values ('Hello-Tom'); -- Fails as expected
Msg 2627 Level 14 State 1 Line 1
Violation of UNIQUE KEY constraint 'uq_clean_data'. Cannot insert duplicate key in object 'dbo.t'. The duplicate key value is (Hello).
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.

select * from t;


a
clean_data


Hello-World
Hello

Theremin
Theremin

-123


Happy
Happy