What Happens to a Trigger with Multiple Affected Rows in SQL Server?

If there is only one row affected by a DML statement, we know that the DML trigger will be executed once. But how many times the DML trigger will be executed if the DML statement resulted multiple affected rows? The answer is still one.

In the case of multiple affected rows, both INSERTED and DELETED tables will contain multiple rows.

If you want your trigger to report all affected rows, you need to write a loop:

USE FyiCenterData;
GO
select * from fyi_users;

-- reporting the first affected row only
UPDATE fyi_users SET email=name;
GO
Email changed from NULL to John King
(5 row(s) affected)

-- reporting all affected rows
ALTER TRIGGER update_user ON fyi_users
AFTER UPDATE
AS
  SELECT 'Email changed from '
    + ISNULL(d.email,'NULL')
    + ' to '
    + ISNULL(i.email,'NULL')
    FROM INSERTED AS i, DELETED AS d
    WHERE i.id = d.id;
GO

UPDATE fyi_users SET email=REVERSE(name);
GO
------------------------------------------------------
Email changed from Marc Kumar to ramuK craM
Email changed from Roy Bush to hsuB yoR
Email changed from Jack Gate to etaG kcaJ
Email changed from Nancy Greenberg to grebneerG ycnaN
Email changed from John King to gniK nhoJ
(5 row(s) affected)

(5 row(s) affected)

⇒ "INSTEAD OF" - Overriding DML Statements with Triggers in SQL Server

⇐ Improving the Trigger to Handle NULL Values in SQL Server

⇑ Creating and Managing Triggers in SQL Server

⇑⇑ SQL Server Database Tutorials