A trigger is a unique store procedure that is executes to give a response to particular action on the table of a database using  the following SQL statements:

1. Data Manipulation Language (DML) SQL Statements (like INSERT, UPDATE or DELETE).
2. A database definition (DDL) statement (CREATE, ALTER, or DROP).
3. A database operation (SERVERERROR, LOGON, LOGOFF, STARTUP, or SHUTDOWN).

Basically, Trigger is like a database object which works with a tables and execute significantly i.e. we can not execute the trigger explicitly.

   We have a two types of trigger:
   1.  After Triggers (For Triggers)--> It is used to executes after DML SQL statements.
    2. Instead Of Triggers --> To executes instead of actual DML statements.

Syntax for creating a Trigger:-

CREATE [OR REPLACE ] TRIGGER trigger_name 
ON table_name 
{AFTER | INSTEAD OF } 
{INSERT [OR] | UPDATE [OR] | DELETE} 
AS
DECLARE
   Declaration-statements
BEGIN 
   Executable-statements
END;

For Example:-

In this example we illustrate how to create a trigger in sql. We are creating a two tables  Employee with the column (EmpID,empName,empSalary and empDesignation) and Department with the column(DeptID,deptName,EmpID). After creating tables we are creating a trigger on the Employee table that will execute after performing an INSERT operations on the Employee table. This trigger will insert a new record in Department table corresponding to the EmpID of a new record inserted in Employee table.

CREATE TRIGGER InsertDepartment
       ON Employee
       AFTER INSERT
AS
BEGIN
       DECLARE @id int
       SELECT @id = max(EmpID) FROM Employee 
       INSERT INTO Departments
       (deptName,EmpID)
       VALUES
       ('Management',@id)
END

To understand the above code how the trigger is working, We need to perform INSERT operation on Employee table.

INSERT INTO Employee (empName,empSalary and empDesignation)
VALUES ('Kritika', 24,7500.00,'HR' );

Now the trigger InsertDepartment will execute automatically and add a new record in Department table.