SQL Server 2008-CAST到nvarchar无法正常工作

Question

I have the following SQL statement as part of a trigger that is not working and cant figure out why:

DECLARE @Orderid INT
DECLARE @CountOfOrderItems INT
DECLARE @EmployeeID INT
DECLARE @EmployeeName NVARCHAR

SELECT @Orderid = DELETED.id

SET @CountOfOrderItems = (select count(*) from OrderDetails where OrderID = @Orderid)

SET @EmployeeName = (SELECT Firstname + ' ' + Lastname from Employees where employees.id = 
                      (SELECT employeeid from Orders where Orders.id = @Orderid))

        INSERT INTO applicationlog (Sourceobject,Logtype,Message)
        VALUES ('Orders table',
                'Order deleted',
                'Order with ' + CAST(@CountOfOrderItems as varchar) + ' items deleted by '+ CAST(@EmployeeName as nvarchar))

The column datatypes for the applicationlog table are all nvarchar(255) for sourceobject and logtype and nvarchar(max) for message.

The issue is, when I include the CAST(@EmployeeName as nvarchar) bit, nothing is entered in the message column, and it shows null. Data is entered in the other two columns. Casting the employeename as varchar also doesn't work.

When I remove the bit shown in bold above and use the following code, it works and the casting for the countoforderitems works.

INSERT INTO applicationlog (Sourceobject,Logtype,Message)
VALUES ('Orders table',
        'Order deleted',
        'Order with ' + CAST(@CountOfOrderItems as varchar) + ' items deleted')

I have no idea why and would appreciate some help, as I need to log the user that is deleting the record.

Thanks

UPDATE

Thanks to all that have responded. I declared employeename as a VARCHAR(255) and edited the insert statement to

        INSERT INTO applicationlog (Sourceobject,Logtype,Message)
    values ('Orders table','Order deleted','Order with ' + CAST(@CountOfOrderItems as varchar(255)) + ' items deleted by ' + coalesce(@EmployeeName,''))

and now all information is there except the @Employeename which i think is returning an empty string, even though the employee exists in the table.So in the last column i now have "3 items deleted by "

Which is better than the null before, but why is the @Employeename blank?

Answer 1

You are using nvarchar (or varchar or nchar or char ) without a length in at least three places. Do not do this. The default length varies by context. So:

DECLARE @EmployeeName NVARCHAR

Is really:

DECLARE @EmployeeName NVARCHAR(1)

And

CAST(@CountOfOrderItems as varchar)

is really:

CAST(@CountOfOrderItems as varchar(30))

(Okay, this is probably ok).

So, be explicit about the lengths. You do not need to cast @EmployeeName for the string concatenation. And, you do not need to use set because you can set the value of a variable in a select statement:

DECLARE @Orderid INT;
DECLARE @CountOfOrderItems INT;
DECLARE @EmployeeID INT;
DECLARE @EmployeeName NVARCHAR(255);

SELECT @Orderid = DELETED.id;  -- This looks strange.  I think you are missing a `from` clause

select  @CountOfOrderItems = count(*)
from OrderDetails
where OrderID = @Orderid);

SELECT @EmployeeName = Firstname + ' ' + Lastname
from Employees
where employees.id = (SELECT employeeid from Orders where Orders.id = @Orderid));

INSERT INTO applicationlog (Sourceobject, Logtype, Message)
    VALUES ('Orders table',
            'Order deleted',
            'Order with ' + CAST(@CountOfOrderItems as varchar(8000)) + ' items deleted by '+ @EmployeeName ;

Or, I would just dispense with all the intermediate stuff and do:

INSERT INTO applicationlog (Sourceobject, Logtype, Message)
    SELECT 'Orders table', 'Order deleted',
           replace(replace('Order with @n items deleted by @by',
                           '@n', count(od.orderid)
                          ), '@by', e.Firstname + ' ' + e.Lastname
                  )
    FROM deleted d JOIN
         orders o
         on d.id = o.orderId JOIN
         employees e
         on o.employeeid = e.employeeid LEFT JOIN
         orderdetails od
         on od.orderid = o.orderid;

I think a single query better shows the intention of what you are doing and you don't need to issue a bunch of queries and assign variables.

Answer 2

It looks like you're mixing datatypes here. If you want it to be nvarchar then it all needs to be Nvarchar. You should also specify the lengths you want like so:

VALUES ('Orders table',
                'Order deleted',
                N'Order with ' + CAST(@CountOfOrderItems as Nvarchar(10)) + N' items deleted by '+ CAST(@EmployeeName as nvarchar(10)))

This all being said I don't know why you are casting @EmployeeName as an Nvarchar when you've already declared it as Nvarchar, it seems a redundant process.

If your @EmployeeName is null it will also show null for the whole string, to get around this you can use the following statement at the top of you query:

SET CONCAT_NULL_YIELDS_NULL OFF