CREATE TRIGGER [dbo].[trgAfterInsert] ON [dbo].[Table_a_aux] 
After Insert
AS
BEGIN

Declare @dDate as Date;
Declare @iTotal as int;

Select 
    @dDate = i.[date],
    @iTotal = i.total
From inserted i;


IF EXISTS (Select [date] from Table_a where [date] = @dDate) 
BEGIN
Update Table_a 
    SET total = total + @iTotal
WHERE
    [date] = @dDate
END
ELSE 
BEGIN
INSERT INTO Table_a ([date],total) Values (@dDate,@iTotal)
END