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Triggers

SQL Server Usage

Triggers are special types of stored procedures that run automatically in response to events. They are most commonly used for Data Manipulation Language (DML).

SQL Server supports AFTER, FOR, and INSTEAD OF triggers, which you can create on tables and views (AFTER and FOR are synonymous). SQL Server also provides an event trigger framework at the server and database levels that includes Data Definition Language (DDL), Data Control Language (DCL), and general system events such as login.

Trigger Run

AFTER triggers runs after DML statements complete run. INSTEAD OF triggers run code in place of the original DML statement. You can create AFTER triggers on tables only. You can create INSTEAD OF triggers on tables and views.

You can create only one INSTEAD OF trigger for any given object and event. When multiple AFTER triggers exist for the same event and object, you can partially set the trigger order by using the sp_settriggerorder system stored procedure. You can use it to set the first and last triggers to be run, but not the order of others.

Trigger Scope

SQL Server supports statement level triggers only. The trigger code runs once for each statement. The data modified by the DML statement is available to the trigger scope and is saved in two virtual tables: INSERTED and DELETED. These tables contain the entire set of changes performed by the DML statement that caused trigger run.

SQL Server triggers always run within the transaction of the statement that triggered the run. If the trigger code issues an explicit ROLLBACK, or causes an exception that mandates a rollback, the DML statement is also rolled back. For INSTEAD OF triggers, the DML statement doesn’t run and doesn’t require a rollback.

Examples

Use a DML trigger to audit invoice deletions

The following examples demonstrate how to use a trigger to log rows deleted from a table.

Create and populate the Invoices table.

CREATE TABLE Invoices
(
  InvoiceID INT NOT NULL PRIMARY KEY,
  Customer VARCHAR(20) NOT NULL,
  TotalAmount DECIMAL(9,2) NOT NULL
);

INSERT INTO Invoices (InvoiceID,Customer,TotalAmount)
VALUES
(1, 'John', 1400.23),
(2, 'Jeff', 245.00),
(3, 'James', 677.22);

Create the InvoiceAuditLog table.

CREATE TABLE InvoiceAuditLog
(
  InvoiceID INT NOT NULL PRIMARY KEY,
  Customer VARCHAR(20) NOT NULL,
  TotalAmount DECIMAL(9,2) NOT NULL,
  DeleteDate DATETIME NOT NULL DEFAULT (GETDATE()),
  DeletedBy VARCHAR(128) NOT NULL DEFAULT (CURRENT_USER)
);

Create an AFTER DELETE trigger to log deletions from the Invoices table to the audit log.

CREATE TRIGGER LogInvoiceDeletes
ON Invoices
AFTER DELETE
AS
BEGIN
INSERT INTO InvoiceAuditLog (InvoiceID, Customer, TotalAmount)
SELECT InvoiceID,
  Customer,
  TotalAmount
FROM Deleted
END;

Delete an invoice.

DELETE FROM Invoices
WHERE InvoiceID = 3;

Query the content of both tables.

SELECT *
FROM Invoices AS I
  FULL OUTER JOIN
  InvoiceAuditLog AS IAG
  ON I.InvoiceID = IAG.InvoiceID;

For the preceding example, the result looks as shown following.

InvoiceID  Customer  TotalAmount  InvoiceID  Customer  TotalAmount  DeleteDate      DeletedBy
1          John      1400.23      NULL       NULL      NULL         NULL            NULL
2          Jeff      245.00       NULL       NULL      NULL         NULL            NULL
NULL       NULL      NULL         3          James     677.22       20180224 13:02  Domain/JohnCortney

Create a DDL trigger

Create a trigger to protect all tables in the database from accidental deletion.

CREATE TRIGGER PreventTableDrop
ON DATABASE FOR DROP_TABLE
AS
BEGIN
  RAISERROR ('Tables can't be dropped in this database', 16, 1)
  ROLLBACK TRANSACTION
END;

Test the trigger by attempting to drop a table.

DROP TABLE [Invoices];
  GO

The system displays the following message explaining that the Invoices table can’t be dropped:

Msg 50000, Level 16, State 1, Procedure PreventTableDrop, Line 5 [Batch Start Line 56]
Tables Can't be dropped in this database.
Msg 3609, Level 16, State 2, Line 57
The transaction ended in the trigger. The batch has been aborted.

For more information, see DML Triggers and DDL Triggers in the SQL Server documentation.

PostgreSQL Usage

Triggers provide much of the same functionality as SQL Server:

Unlike SQL Server triggers, PostgreSQL triggers must call a function. They don’t support anonymous blocks of PL/pgSQL code as part of the trigger body. The user-supplied function is declared with no arguments and has a return type of trigger.

PostgreSQL DML Triggers

PostgreSQL triggers can be fired BEFORE or AFTER a DML operation.

PostgreSQL triggers can run INSTEAD OF a DML command when created on views.

PostgreSQL triggers can run FOR EACH ROW affected by the DML statement or FOR EACH STATEMENT running only once as part of a DML statement.

PostgreSQL Event Triggers

An event trigger runs when a specific event associated with the trigger occurs in the database. Supported events include ddl_command_start, ddl_command_end, table_rewrite, and sql_drop.

For a full list of supported PostgreSQL event trigger types, see Event Trigger Firing Matrix in the PostgreSQL documentation.

PostgreSQL CREATE TRIGGER Synopsis

CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ]}
  ON table_name
  [ FROM referenced_table_name ]
  [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
  [ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ]
  [ FOR [ EACH ] { ROW | STATEMENT } ]
  [ WHEN ( condition ) ]
  EXECUTE PROCEDURE function_name ( arguments )

where event can be one of:

  INSERT
  UPDATE [ OF column_name [, ... ] ]
  DELETE
  TRUNCATE

Examples

Create a trigger

Create a trigger function that stores the run logic (this is the same as a SQL Server DML trigger).

CREATE OR REPLACE FUNCTION PROJECTS_SET_NULL()
  RETURNS TRIGGER
  AS $$
  BEGIN
IF TG_OP = 'UPDATE' AND OLD.PROJECTNO != NEW.PROJECTNO OR
  TG_OP = 'DELETE' THEN
UPDATE EMP
  SET PROJECTNO = NULL
  WHERE EMP.PROJECTNO = OLD.PROJECTNO;
  END IF;
  IF TG_OP = 'UPDATE' THEN RETURN NULL;
    ELSIF TG_OP = 'DELETE' THEN RETURN NULL;
  END IF;
END; $$
LANGUAGE PLPGSQL;

CREATE FUNCTION

Create the trigger.

CREATE TRIGGER TRG_PROJECTS_SET_NULL
AFTER UPDATE OF PROJECTNO OR DELETE
ON PROJECTS
FOR EACH ROW
EXECUTE PROCEDURE PROJECTS_SET_NULL();

CREATE TRIGGER

Test the trigger by deleting a row from the PROJECTS table.

DELETE FROM PROJECTS WHERE PROJECTNO=123;
SELECT PROJECTNO FROM EMP WHERE PROJECTNO=123;

projectno
(0 rows)

Create a trigger

Create an event trigger function. This is the same as a SQL Server DDL System/Schema level trigger, such as a trigger that prevents running a DDL DROP on objects in the HR schema.

Note that trigger functions are created with no arguments and must have a return type of TRIGGER or EVENT_TRIGGER.

CREATE OR REPLACE FUNCTION ABORT_DROP_COMMAND()
  RETURNS EVENT_TRIGGER
  AS $$
BEGIN
  RAISE EXCEPTION 'The % Command is Disabled', tg_tag;
END; $$
LANGUAGE PLPGSQL;

CREATE FUNCTION

Create the event trigger, which runs before the start of a DDL DROP command.

CREATE EVENT TRIGGER trg_abort_drop_command
  ON DDL_COMMAND_START
  WHEN TAG IN ('DROP TABLE', 'DROP VIEW', 'DROP FUNCTION', 'DROP
    SEQUENCE', 'DROP MATERIALIZED VIEW', 'DROP TYPE')
  EXECUTE PROCEDURE abort_drop_command();

Test the trigger by attempting to drop the EMPLOYEES table.

DROP TABLE EMPLOYEES;

ERROR: The DROP TABLE Command is Disabled
CONTEXT: PL/pgSQL function abort_drop_command() line 3 at RAISE

Summary

For more information, see Trigger Functions in the PostgreSQL documentation.