SQL Assign datediff to table column in insert trigger - sql

Question

Table
CREATE TABLE CurrentApplication
(
StartDate datetime NOT NULL,
EndDate datetime NOT NULL,
NoOfDays integer,
StaffID integer NOT NULL,
AppStatus varchar(30) NOT NULL DEFAULT 'PENDING'
)
Insert Trigger
CREATE TRIGGER InsertNoOfDays ON CurrentApplication
AFTER INSERT
BEGIN
INSERT INTO CurrentApplication
NoOfDays AS Datediff(day, StartDate, EndDate)
END
I have NoOfDays column which should hold the DateDiff between StartDate and EndDate and the value should be inserted whenever a new record is inserted into the table. How do I write the trigger and do it? I've tried but my trigger doesn't work. Thanks!

Answer 1

One method is to use an INSTEAD OF trigger:
CREATE TRIGGER InsertNoOfDays ON CurrentApplication
INSTEAD OF INSERT AS
BEGIN
INSERT INTO CurrentApplication( . . ., NoOfDays)
SELECT . . .,
Datediff(day, StartDate, EndDate)
FROM inserted;
END;

Answer 2

One thing you can do is add a computed column instead of actual column as NoOfdays.
There are benefits and drawbacks in adding a computed column.
You don't have have a trigger and calculate it. Saves on writes.
This also means sometimes you cannot index on that column and reads may have an impact.
Here is the documentation from Microsoft on computed columns (documentation says it is only on SQL 2016 onwards):
Specify Computed Columns in Tables
But, here is sample in SQL 2014 that worked for me (I think it works in SQL 2012 also)
use tempdb
GO
/*
Wors in version:
Microsoft SQL Server 2014 - 12.0.2000.8 (X64)
Standard Edition (64-bit
*/
IF OBJECT_ID('tempdb.dbo.CurrentApplication') IS NULL
CREATE TABLE dbo.CurrentApplication
(
StartDate datetime NOT NULL,
EndDate datetime NOT NULL,
NoOfDays AS (datediff(dd, StartDate, EndDate)) PERSISTED,
StaffID integer NOT NULL,
)
INSERT INTO dbo.CurrentApplication(StartDate, EndDate, StaffId)
SELECT TOP 10
StartDate = DATEADD(dd, object_id, '1/1/2017')
,EndDate = DATEADD(dd, object_id*object_id, '1/1/2017')
,StaffId = object_id
FROM sys.objects
WHERE object_id < 300
order by Object_id
SELECT * FROM CurrentApplication
IF OBJECT_ID('tempdb.dbo.CurrentApplication') IS NOT NULL
DROP TABLE dbo.CurrentApplication

Answer 3

Since not eligible to add comments, adding as a new answer.
Adding to #Gordon Linoff's answer, just substituted column names instead of periods to see if that caused the error you getting. I didnt get that error. Not sure if there is anything with SQL 2012.
But, this works (in SQL 2014):
CREATE TRIGGER InsertNoOfDays ON dbo.CurrentApplication
INSTEAD OF INSERT AS
BEGIN
INSERT INTO CurrentApplication(StartDate, EndDate, NoOfDays, StaffID)
SELECT StartDate
,EndDate
,Datediff(dd, StartDate, EndDate)
,StaffId
FROM inserted as i;
END;