T-SQL conditional UPDATE (v2)

I have a table:

Message (MessageID int, Subject nvarchar(100), Body nvarchar(max))

After a message is being updated on UI, I call a stored proc to update that table. In some cases user might update just subject, in other cases just body. I want this stored proc to only update what has changed, so I'm also passing flags showing whether subject or body has been updated:

create proc UpdateMessage(
  @MessageID int, 
  @Subject nvarchar(100), 
  @Body nvarchar(max),
  @SubjectChanged bit,
  @BodyChanged bit)

And now i'm confused how to build the conditional UPDATE statement. My first thought was to use CASE:

Update [Message] 
SET 
CASE WHEN @SubjectChanged = 1 THEN [Subject] = @Subject ELSE 1=1 END,
CASE WHEN @BodyChanged = 1 THEN Body = @Body ELSE 1=1 END,
WHERE MessageID = @MessageID

... but that doesn't seem to be a correct syntax as CASE has to be the right side 开发者_StackOverflowof an assigment.

Any ideas how I could do that? (And keep in mind that in reality there are 6 parameters that can be updated, not two)

The syntax required to create your statement is:

Update [Message] 
SET    [Subject] = CASE WHEN @SubjectChanged = 1 THEN @Subject ELSE [Subject] END,
       Body = CASE WHEN @BodyChanged = 1 THEN @Body ELSE Body END
WHERE  MessageID = @MessageID

if you still want to stick to it after all the suggestions.

N.b. if you leave out the ELSE [Subject] part of the CASE statements, instead of ignoring the UPDATE it sets the field to NULL.

update Message set
    Subject = (case when @SubjectChanged = 1 then @Subject else Subject end),
    Body = (case when @BodyChanged = 1 then @Body else Body end)

where MessageID = @MessageID

That should really be all you need. However, if you truly can't update the field if it hasn't changed, then you'll have to do it in separate statements.

if @SubjectChanged = 1 
    update Message set Subject = @Subject where MessageID = @MessageID
if @BodyChanged = 1 
    update Message set Body = @Body where MessageID = @MessageID

Your best bet, by far, is to use explicit IF statements:

IF @subjectHasChanged = 1 and @bodyHasChanged = 1
 UPDATE Messages SET Subject = @subject, Body = @body 
   WHERE MessageId = @MessageId
ELSE IF @subjectHasChanged = 1
 UPDATE Messages SET Subject = @subject WHERE MessageId = @MessageId
ELSE IF @bodyHasChanged
 UPDATE Messages SET Body = @body WHERE MessageId = @MessageId

From a performance point of view, nothing beats this. Because SQL can see during query compilation that you only update Body, or Subject, or both, it can generate the appropriate plan, for instance not even bothering to open (for update) the non-clustered index you have on Subject (if you have one, of course) when you only update Body.

From a code code quality point of view, this is disaster, a nightmare to maintain. But acknowledging the problem is 80% solving the problem :) . You can use code generation techniques for instance to maintain such problem procedures.

Another viable approach is actually to use dynamic SQL, construct the UPDATE in the procedure and use sp_executesql. It has its own set of problems, as all dynamic SQL has. There are resources about dynamic SQL problems, and there are workarounds and solutions, see The Curse and Blessings of Dynamic SQL.

Seems to me like you are wasting a lot of effort. If you retrieve the six values, display them to the user (in some user interface) and they can change some variable number of them and hit a "save" button - then just update all 6 fields every time, getting the new values from the user input fields.

Some may not have changed, but so what. Much simpler code that way.

Use DEFAULT values for the stored procedure parameters.

create proc UpdateMessage(
  @MessageID int,  -- mandatory
  @Subject nvarchar(100) = NULL, 
  @Body nvarchar(max) = NULL)

Then, you can structure your update in this way:

Update [Message] 
SET 
[Subject] = ISNULL(@Subject, [Subject]),
Body = ISNULL(@Body, Body)
WHERE MessageID = @MessageID

CREATE PROCEDURE UpdateMessage
  @MessageID int, 
  @Subject nvarchar(100), 
  @Body nvarchar(max),
AS
BEGIN
    if(@Subject is null or @Subject='')
        SELECT @Subject=Subject FROM Message WHERE MessageID=@MessageID
    if(@Body is null or @Body='')
        SELECT @Body=Body FROM Message WHERE MessageID=@MessageID
    UPDATE Message SET Subject=@Subject, Body=@Body WHERE MessageID=@MessageID
END
GO

I am not sure if this is the best way to do it, but maybe you can try

IF @SubjectChanged = 1 THEN
   BEGIN
      UPDATE [Message]
      SET [Subject] = @Subject
      WHERE MessageID = @MessageID     
   END
END

IF @BodyChanged = 1 THEN
   BEGIN
      UPDATE [Message]
      SET Body = @Body
      WHERE MessageID = @MessageID
   END
END

I would highly recommend using Adam Robinson's method if you require this to be in a single stored procedure.

Even better would be to simply use separate stored procedures for each one of these updates.