Error Level 16 indicates that the error can be corrected by the user.

From what you have stated, I am led to believe that there is more to your problem than what you have posted. There is no need to use a trigger to enforce your requirement.

For example your UPDATE T-SQL code could be as simple as:

DECLARE @U VARCHAR(1)

SET @U = 'A'

UPDATE #RB SET X = @U WHERE X <> 'b'

The fallacy of the simple code is that if the value is NOT 'B', but lets say it is 'Z'. then the above T-SQL would update the 'Z' to an 'A'.

To get tested help please post table definition, sample data and required results when using the sample data. To do so quickly and easily please read the article in the first link of my signature block.