Parameter truncation doesn't raise error in stored proc

I can't get my stored proc to return an error when the value passed to the parameter is larger than the parameter's definition and larger than the field definition.  If I execute the SQL in the query window, an error is returned.

Table definition:
CREATE TABLE Employee(Eid int, Fname varchar(20), Lname varchar(20))

Populate 1 record:
  INSERT INTO Employee (EId, FName, LName) VALUES (1,'Ford','Perfect')

Update it from Query Analyzer
  UPDATE Employee  
  SET fname = 'FName', Lname = 'LNamefjdkfjkdfjkdjfkdjfk'

This raises the following error:
  Server: Msg 8152, Level 16, State 9, Line 1
  String or binary data would be truncated.
  The statement has been terminated.

Here is the update statement converted into a stored proc:

CREATE PROC updateEmployee3 (
      @FName varchar(10)
      , @LName varchar(10))
AS  
  UPDATE Employee  
  SET  fname = @Fname, lname = @Lname

RETURN @@ERROR  
 
When I call the stored proc in query window as follows, the value for FName is truncated but no error is raised.  Why?
  exec updateEmployee3 'ABCDEFGHIJKLM', 'LName'

I've tried this with other types of errors, e.g., assigning NULL to a field defined as NOT NULL, assigning a string to a int field, and in those cases the appropriate error is returned.
LVL 9
dancebertAsked:
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danblakeCommented:
Having a look here:

CREATE PROC updateEmployee3 (
     @FName varchar(10)
     , @LName varchar(10))
AS  
  select @FName,@LName

  UPDATE Employee  
  SET  fname = @Fname, lname = @Lname

SELECT @@ERROR, @@ROWCOUNT

===============================

Running the following QA (Command):
updateEmployee3 'FName','LNamefjdkfjkdfjkdjfkdjfk'

You will notice that when the vars are passed to the SP, they are truncated on input.  They will not be trapped within the @@Error value.

Input vars into SPs are automatically truncated (without error) on input this is due to no checking when passing values to variables on variable length and if exceeding input length -- generate an error.  This is different than inputing into a table.

The following demonstrates this:
DECLARE @find varchar(3)
SET @find = 'Ring'
select @find

Output
---------------------------------------------------------------
Rin
0
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