Question

I’m working on a trigger which needs to re-insert its data on another table.

The destination table has a primary key INT NOT NULL WITHOUT Identity, so I have 2 choices:

1. Calculate the maximum and insert from here.
2. Take max value from a sequences table.

I use to always create a table variable with identity and insert shifting from the value calculated before.

CREATE TRIGGER trg
   ON  [dbo].[table] 
   AFTER INSERT
AS
BEGIN
    SET NOCOUNT ON;

    DECLARE @t TABLE (sec INT IDENTITY(1, 1), id INT)
    DECLARE @ini INT

    SELECT @ini = ISNULL(MAX(id), 0) FROM tableDest
    -- SELECT @ini = value FROM sequencesTable WHERE seqId = 987

    INSERT INTO @t (id) SELECT id FROM inserted

    INSERT INTO tableDest 
        (id, field1, field2) 
    SELECT @ini + t.sec, field1, field2
    FROM @t t
        JOIN inserted ON t.id = inserted.id


    -- SELECT @ini = @ini + MAX(t.sec) FROM @t
    -- UPDATE sequencesTable SET value = @ini WHERE seqId = 987
END

Is it any better way to do this?

Thanks in Advance.

Answer 1

Assuming SQL 2005+, you can use ROW_NUMBER():

CREATE TRIGGER trg
   ON  [dbo].[table] 
   AFTER INSERT
AS
BEGIN
    SET NOCOUNT ON;

    INSERT INTO tableDest
        (id, field1, field2)
    SELECT
        Seed.Value + ROW_NUMBER() OVER(ORDER BY Id), 
        field1, 
        field2
    FROM Inserted
    CROSS JOIN (
       SELECT MAX(id) as Value
       FROM tableDest
    ) as Seed
END

Doing a CROSS JOIN instead of getting the seed value directly saves you from the concurrency headache of MAX(Id) changing between getting the value and inserting it. Otherwise, you’d need a SERIALIZABLE transaction to prevent new rows from being inserted into tableDest after you read it.