SQL Server Update Trigger, Get Only modified fields

Question

I am aware of COLUMNS_UPDATED, well I need some quick shortcut (if anyone has made, I am already making one, but if anyone can save my time, I will appriciate it)

I need basicaly an XML of only updated column values, I need this for replication purpose.

SELECT * FROM inserted gives me each column, but I need only updated ones.

something like following...

CREATE TRIGGER DBCustomers_Insert
    ON DBCustomers
    AFTER UPDATE
AS
BEGIN
    DECLARE @sql as NVARCHAR(1024);
    SET @sql = 'SELECT ';


    I NEED HELP FOR FOLLOWING LINE ...., I can manually write every column, but I need 
    an automated routin which can work regardless of column specification
    for each column, if its modified append $sql = ',' + columnname...

    SET @sql = $sql + ' FROM inserted FOR XML RAW';

    DECLARE @x as XML;
    SET @x = CAST(EXEC(@sql) AS XML);


    .. use @x

END

Answer 1

The only way that occurs to me that you could accomplish this without hard coding column names would be to drop the contents of the deleted table to a temp table, then build a query based on the table definition to to compare the contents of your temp table and the actual table, and return a delimited column list based on whether they do or do not match. Admittedly, the below is elaborate.

Declare @sql nvarchar(4000)
DECLARE @ParmDefinition nvarchar(500)
Declare @OutString varchar(8000)
Declare @tbl sysname

Set @OutString = ''
Set @tbl = 'SomeTable' --The table we are interested in
--Store the contents of deleted in temp table
Select * into #tempDelete from deleted 
--Build sql string based on definition 
--of table 
--to retrieve the column name
--or empty string
--based on comparison between
--target table and temp table
set @sql = ''
Select @sql = @sql + 'Case when IsNull(i.[' + Column_Name + 
'],0) = IsNull(d.[' + Column_name + '],0) then '''' 
 else ' + quotename(Column_Name, char(39)) + ' + '',''' + ' end +'
from information_schema.columns 
where table_name = @tbl
--Define output parameter
set @ParmDefinition = '@OutString varchar(8000) OUTPUT'
--Format sql
set @sql = 'Select @OutString = ' 
+ Substring(@sql,1 , len(@sql) -1) + 
' From SomeTable i  ' --Will need to be updated for target schema
+ ' inner join #tempDelete d on
i.PK = d.PK ' --Will need to be updated for target schema
--Execute sql and retrieve desired column list in output parameter
exec sp_executesql @sql, @ParmDefinition, @OutString OUT
drop table  #tempDelete
--strip trailing column if a non-zero length string 
--was returned
if Len(@Outstring) > 0 
    Set @OutString = Substring(@OutString, 1, Len(@Outstring) -1)
--return comma delimited list of changed columns. 
Select @OutString 
End

Answer 2

Inside the trigger, you can use COLUM_UPDATED like this in order to get updated value

-- Get the table id of the trigger
--
DECLARE @idTable      INT

SELECT  @idTable = T.id 
FROM    sysobjects P JOIN sysobjects T ON P.parent_obj = T.id 
WHERE   P.id = @@procid

-- Get COLUMNS_UPDATED if update
--
DECLARE @Columns_Updated VARCHAR(50)

SELECT  @Columns_Updated = ISNULL(@Columns_Updated + ', ', '') + name 
FROM    syscolumns 
WHERE   id = @idTable   
AND     CONVERT(VARBINARY,REVERSE(COLUMNS_UPDATED())) & POWER(CONVERT(BIGINT, 2), colorder - 1)  > 0

But this snipet of code fails when you have a table with more than 62 columns.. Arth.Overflow...

Here is the final version which handles more than 62 columns but give only the number of the updated columns. It's easy to link with 'syscolumns' to get the name

DECLARE @Columns_Updated = VARCHAR(100)
SET     @Columns_Updated = ''   

DECLARE @maxByteCU INT
DECLARE @curByteCU INT
SELECT  @maxByteCU = DATALENGTH(COLUMNS_UPDATED()), 
        @curByteCU = 1

WHILE @curByteCU <= @maxByteCU BEGIN
    DECLARE @cByte INT
    SET     @cByte = SUBSTRING(COLUMNS_UPDATED(), @curByteCU, 1)

    DECLARE @curBit INT
    DECLARE @maxBit INT
    SELECT  @curBit = 1, 
            @maxBit = 8
    WHILE @curBit <= @maxBit BEGIN
        IF CONVERT(BIT, @cByte & POWER(2,@curBit - 1)) <> 0 
            SET @Columns_Updated = @Columns_Updated + '[' + CONVERT(VARCHAR, 8 * (@curByteCU - 1) + @curBit) + ']'
        SET @curBit = @curBit + 1
    END
    SET @curByteCU = @curByteCU + 1
END