Question

I want to throw an exception if cardescription is in table in row with specific @idCar

cardescription is not PK, and any change in design is not allowed

insert into carsdescription
  (description) 
value 
  (@description,@idCar)

Examples:

IDCar  | Description
---------------------
1      | nice        - this record was in table before.
1      | nice        - should throw exception
2      | nice        - it is allowed

Carsdescription table:

• ID (PK, int, autoincrement)
• idcar(FK, int)
• Description(varchar)

Cars table:

• Id(int, PK, autoincrement)
• Name(nvarchar(255)

Answer 1

You should create a unique index over IDCar and Description. Then your Database will prevent inserting duplicate data. Your CommandObject will then throw an exception.

insert into carsdescription(IDCar, description) 
values (@idCar, @description)

But to be nice to your users you should first lookup in your table if a duplicate entry would be insterted.

select count(*) from  carsdescription where IDCar = @idCar and decription = @description

EDIT:

Fore some reasons schema changes are not allowed. This means, your Database is unable to protected you against duplicate entries. So, as Matthew pointed out correctly, you have to:

• First: do the select statement. If count is greater than 0, throw your Exception
• Second: Do the insert