Insert Update trigger how to determine if insert o

Question

I need to write an Insert, Update Trigger on table A which will delete all rows from table B whose one column (say Desc) has values like the value inserted/updated in the table A's column (say Col1). How would I go around writing it so that I can handle both Update and Insert cases. How would I determine if the trigger is executed for an update or insert.

Answer 1

CREATE TRIGGER dbo.TableName_IUD
ON dbo.TableName
AFTER INSERT, UPDATE, DELETE
AS 
BEGIN
    SET NOCOUNT ON;

    --
    -- Check if this is an INSERT, UPDATE or DELETE Action.
    -- 
    DECLARE @action as char(1);

    SET @action = 'I'; -- Set Action to Insert by default.
    IF EXISTS(SELECT * FROM DELETED)
    BEGIN
        SET @action = 
            CASE
                WHEN EXISTS(SELECT * FROM INSERTED) THEN 'U' -- Set Action to Updated.
                ELSE 'D' -- Set Action to Deleted.       
            END
    END
    ELSE 
        IF NOT EXISTS(SELECT * FROM INSERTED) RETURN; -- Nothing updated or inserted.

    ...

    END

Answer 2

This might be a faster way:

DECLARE @action char(1)

IF COLUMNS_UPDATED() > 0 -- insert or update
BEGIN
    IF EXISTS (SELECT * FROM DELETED) -- update
        SET @action = 'U'
    ELSE
        SET @action = 'I'
    END
ELSE -- delete
    SET @action = 'D'

Answer 3

Many of these suggestions do not take into account if you run a delete statement that deletes nothing.
Say you try to delete where an ID equals some value that does not exist in the table.
Your trigger still gets called but there is nothing in the Deleted or Inserted tables.

Use this to be safe:

--Determine if this is an INSERT,UPDATE, or DELETE Action or a "failed delete".
DECLARE @Action as char(1);
    SET @Action = (CASE WHEN EXISTS(SELECT * FROM INSERTED)
                         AND EXISTS(SELECT * FROM DELETED)
                        THEN 'U'  -- Set Action to Updated.
                        WHEN EXISTS(SELECT * FROM INSERTED)
                        THEN 'I'  -- Set Action to Insert.
                        WHEN EXISTS(SELECT * FROM DELETED)
                        THEN 'D'  -- Set Action to Deleted.
                        ELSE NULL -- Skip. It may have been a "failed delete".   
                    END)

Special thanks to @KenDog and @Net_Prog for their answers.
I built this from their scripts.

Answer 4

I've used those exists (select * from inserted/deleted) queries for a long time, but it's still not enough for empty CRUD operations (when there're no records in inserted and deleted tables). So after researching this topic a little bit I've found more precise solution:

declare
    @columns_count int = ?? -- number of columns in the table,
    @columns_updated_count int = 0

-- this is kind of long way to get number of actually updated columns
-- from columns_updated() mask, it's better to create helper table
-- or at least function in the real system
with cte_columns as (
    select @columns_count as n
    union all
    select n - 1 from cte_columns where n > 1
), cte_bitmasks as (
    select
        n,
        (n - 1) / 8 + 1 as byte_number,
        power(2, (n - 1) % 8) as bit_mask
    from cte_columns
)
select
    @columns_updated_count = count(*)
from cte_bitmasks as c
where
    convert(varbinary(1), substring(@columns_updated_mask, c.byte_number, 1)) & c.bit_mask > 0

-- actual check
if exists (select * from inserted)
    if exists (select * from deleted)
        select @operation = 'U'
    else
        select @operation = 'I'
else if exists (select * from deleted)
    select @operation = 'D'
else if @columns_updated_count = @columns_count
    select @operation = 'I'
else if @columns_updated_count > 0
    select @operation = 'U'
else
    select @operation = 'D'

It's also possible to use columns_updated() & power(2, column_id - 1) > 0 to see if the column is updated, but it's not safe for tables with big number of columns. I've used a bit complex way of calculating (see helpful article below).

Also, this approach will still incorrectly classifies some updates as inserts (if every column in the table is affected by update), and probably it will classifies inserts where there only default values are inserted as deletes, but those are king of rare operations (at lease in my system they are). Besides that, I don't know how to improve this solution at the moment.

• COLUMNS_UPDATED() for audit triggers by Piotr Rodak
• Dealing with very large bitmasks by Adam Machanic

Answer 5

After a lot of searching I could not find an exact example of a single SQL Server trigger that handles all (3) three conditions of the trigger actions INSERT, UPDATE, and DELETE. I finally found a line of text that talked about the fact that when a DELETE or UPDATE occurs, the common DELETED table will contain a record for these two actions. Based upon that information, I then created a small Action routine which determines why the trigger has been activated. This type of interface is sometimes needed when there is both a common configuration and a specific action to occur on an INSERT vs. UPDATE trigger. In these cases, to create a separate trigger for the UPDATE and the INSERT would become maintenance problem. (i.e. were both triggers updated properly for the necessary common data algorithm fix?)

To that end, I would like to give the following multi-trigger event code snippet for handling INSERT, UPDATE, DELETE in one trigger for an Microsoft SQL Server.

CREATE TRIGGER [dbo].[INSUPDDEL_MyDataTable]
ON [dbo].[MyDataTable] FOR INSERT, UPDATE, DELETE
AS 

-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with caller queries SELECT statements.
-- If an update/insert/delete occurs on the main table, the number of records affected
-- should only be based on that table and not what records the triggers may/may not
-- select.
SET NOCOUNT ON;

--
-- Variables Needed for this Trigger
-- 
DECLARE @PACKLIST_ID varchar(15)
DECLARE @LINE_NO smallint
DECLARE @SHIPPED_QTY decimal(14,4)
DECLARE @CUST_ORDER_ID varchar(15)
--
-- Determine if this is an INSERT,UPDATE, or DELETE Action
-- 
DECLARE @Action as char(1)
DECLARE @Count as int
SET @Action = 'I' -- Set Action to 'I'nsert by default.
SELECT @Count = COUNT(*) FROM DELETED
if @Count > 0
    BEGIN
        SET @Action = 'D' -- Set Action to 'D'eleted.
        SELECT @Count = COUNT(*) FROM INSERTED
        IF @Count > 0
            SET @Action = 'U' -- Set Action to 'U'pdated.
    END

if @Action = 'D'
    -- This is a DELETE Record Action
    --
    BEGIN
        SELECT @PACKLIST_ID =[PACKLIST_ID]
                    ,@LINE_NO = [LINE_NO]
        FROM DELETED

        DELETE [dbo].[MyDataTable]
        WHERE [PACKLIST_ID]=@PACKLIST_ID AND [LINE_NO]=@LINE_NO
    END
 Else
    BEGIN
            --
            -- Table INSERTED is common to both the INSERT, UPDATE trigger
            --
            SELECT @PACKLIST_ID =[PACKLIST_ID]
                ,@LINE_NO = [LINE_NO]
                ,@SHIPPED_QTY =[SHIPPED_QTY]
                ,@CUST_ORDER_ID = [CUST_ORDER_ID]
            FROM INSERTED 

         if @Action = 'I'
            -- This is an Insert Record Action
            --
            BEGIN
                INSERT INTO [MyChildTable]
                    (([PACKLIST_ID]
                    ,[LINE_NO]
                    ,[STATUS]
                VALUES
                    (@PACKLIST_ID
                    ,@LINE_NO
                    ,'New Record'
                    )
            END
        else
            -- This is an Update Record Action
            --
            BEGIN
                UPDATE [MyChildTable]
                    SET [PACKLIST_ID] = @PACKLIST_ID
                          ,[LINE_NO] = @LINE_NO
                          ,[STATUS]='Update Record'
                WHERE [PACKLIST_ID]=@PACKLIST_ID AND [LINE_NO]=@LINE_NO
            END
    END   

Answer 6

I believe nested ifs a little confusing and:

Flat is better than nested [The Zen of Python]

;)

DROP TRIGGER IF EXISTS AFTER_MYTABLE

GO

CREATE TRIGGER dbo.AFTER_MYTABLE ON dbo.MYTABLE AFTER INSERT, UPDATE, DELETE 

AS BEGIN 

    --- FILL THE BEGIN/END SECTION FOR YOUR NEEDS.

    SET NOCOUNT ON;

    IF EXISTS(SELECT * FROM INSERTED)  AND EXISTS(SELECT * FROM DELETED) 
        BEGIN PRINT 'UPDATE' END 
    ELSE IF EXISTS(SELECT * FROM INSERTED)  AND NOT EXISTS(SELECT * FROM DELETED) 
        BEGIN PRINT 'INSERT' END 
    ELSE IF    EXISTS(SELECT * FROM DELETED) AND NOT EXISTS(SELECT * FROM INSERTED)
        BEGIN PRINT 'DELETED' END
    ELSE BEGIN PRINT 'NOTHING CHANGED'; RETURN; END  -- NOTHING

END