Table Contraints

Is it possible to apply a constraint or equivalent to a table that prevents a record being inserted when the record to be inserted already exists but the values are swapped around?

For example, if a table had two columns: FirstName and SecondName having a record FirstName = ‘John’ and SecondName = ‘Smith’ – then when another record having FirstName =’ Smith’ and SecondName =’ John’ was about to be inserted SQL would reject it.

Good luck!
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Jim HornMicrosoft SQL Server Developer, Architect, and AuthorCommented:
Yep.  You can create an insert trigger to pull this off, that goes something like this:  
CREATE TRIGGER trigger_name ON your_table
IF EXISTS (SELECT * FROM your_table WHERE inserted.first_name = your_table.last_name AND inserted.last_name = your_table.first_name)   
	RAISERROR ('Cant insert a row where the first and last names already exist, but in opposite order.', 16, 1);

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Scott PletcherSenior DBACommented:
If you want to avoid a trigger, you could also create a PERSISTED column with a unique index on it.  I can give you details on that if you'd like them.
antonmsAuthor Commented:
If possible I would prefer not to use a trigger - more info on the persisted column option would be great thanks.
Scott PletcherSenior DBACommented:
ALTER TABLE dbo.tablename
ADD FirstName_SecondName AS CASE WHEN FirstName < SecondName THEN FirstName ELSE ISNULL(SecondName, '') END + CASE WHEN FirstName < SecondName THEN SecondName ELSE ISNULL(FirstName, '') END PERSISTED

ALTER TABLE dbo.tablename
ADD CONSTRAINT tablename__UQ_FirstName_SecondName UNIQUE NONCLUSTERED ( FirstName_SecondName )

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