In databases, triggers are the statements/code which gets executed whenever an event occurred. Once you create a trigger to a particular event on a table, the code specified in the trigger gets executed each time the event occurs. You can create multiple triggers on a single table.

This chapter teaches you how to create and drop triggers using Apache Derby.

Creating a trigger

You can create a trigger in Derby using the CREATE TRIGGER statement.

Syntax

Following is the syntax of the CREATE TRIGGER query.

CREATE TRIGGER trigger_name
{ NO CASCADE BEFORE | AFTER }
{INSERT [OR] | UPDATE [OR] | DELETE}[OF col_name]
ON table_name
[REFERENCING OLD AS o NEW AS n]
[FOR EACH ROW]
Statement

Example

Suppose, we have created a table named Emp in Derby as shown below.

CREATE TABLE Emp (
   Id INT NOT NULL,
   Name VARCHAR(255),
   Salary INT NOT NULL,
   Location VARCHAR(255) 
);

And inserted 5 rows in it.

INSERT INTO Emp(Id, Name, Salary, Location) VALUES
   (1, ''Amit'', 30000, ''Hyderabad''), 
   (2, ''Kalyan'', 40000, ''Vishakhapatnam''),
   (3,''Renuka'', 50000, ''Delhi''), 
   (4, ''Archana'', 15000, ''Mumbai''), 
   (5, ''Trupthi'', 45000, ''Kochin'');

If we have another table named BackUp and our intention is to store deleted rows from Emp table in this.

CREATE TABLE BackUp (
   Id INT NOT NULL,
   Name VARCHAR(255),
   Salary INT NOT NULL,
   Location VARCHAR(255)
);

The following query creates a trigger on the DELETE query table named Emp. It stores the deleted rows of Emp to the table Backup.

ij> CREATE TRIGGER my_trigger
AFTER DELETE ON Emp
REFERENCING OLD AS oldRow
FOR EACH ROW MODE DB2SQL
INSERT INTO BackUp
VALUES (oldRow.Id, oldRow.Name, oldRow.Salary, oldRow.Location);

Now, delete a row from Emp table as −

ij> Delete From Emp where Name = ''Kalyan
1 row inserted/updated/deleted
ij> Delete From Emp where Name = ''Amit
1 row inserted/updated/deleted

If you verify the BackUp table, you can observe the deleted rows in it.

ij> select * from BackUp;
ID |NAME   |SALARY |LOCATION
-------------------------------------------------------------------------
2  |Kalyan |40000  |Vishakhapatnam
1  |Amit   |30000  |Hyderabad

2 rows selected

Deleting a trigger

You can delete a trigger in Derby using the DROP TRIGGER statement.

Syntax

Following is the syntax of the DROP TRIGGER query −

ij> Drop trigger tigger_name;

Example

Following example deletes the trigger my_trigger created above −

ij> Drop trigger my_trigger;
0 rows inserted/updated/deleted

JDBC example

Following JDBC program creates and deletes triggers in Derby.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class Triggers_Example {
   public static void main(String args[]) throws SQLException, ClassNotFoundException {
      //Registering the driver
      Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
      //Getting the Connection object
      String URL = "jdbc:derby:TestDataBase;create=true";
      Connection conn = DriverManager.getConnection(URL);

      //Creating the Statement object
      Statement stmt = conn.createStatement();

      //Creating the Emp table
      stmt.execute("CREATE TABLE Emp ( "
         + "Id INT NOT NULL, "
         + "Name VARCHAR(255), "
         + "Salary INT NOT NULL, "
         + "Location VARCHAR(255))");

      //Insert values in to the EMp table
      String query = "INSERT INTO Emp(Id, Name, Salary, Location) VALUES rn"
         +"(1, ''Amit'', 30000, ''Hyderabad''), "
         + "(2, ''Kalyan'', 40000, ''Vishakhapatnam''), "
         + "(3,''Renuka'', 50000, ''Delhi''), "
         + "(4, ''Archana'', 15000, ''Mumbai''), "
         + "(5, ''Trupthi'', 45000, ''Kochin'')";
         stmt.execute(query);

      //Creating the BackUp table
      stmt.execute("CREATE TABLE BackUp ( "
         + "Id INT NOT NULL, "
         + "Name VARCHAR(255), "
         + "Salary INT NOT NULL, "
         + "Location VARCHAR(255))");

      //Creating a trigger
      String createTrigger = "CREATE TRIGGER my_trigger "
         + "AFTER DELETE ON Emp "
         + "REFERENCING OLD AS oldRow "
         + "FOR EACH ROW MODE DB2SQL "
         + "INSERT INTO BackUp "
         + "VALUES (oldRow.Id, oldRow.Name, oldRow.Salary, oldRow.Location)";
      stmt.execute(createTrigger);
      System.out.println("Trigger created");

      //Deleting records from Emp table
      stmt.executeUpdate("Delete From Emp where Name = ''Kalyan''");
      stmt.executeUpdate("Delete From Emp where Name = ''Amit''");
      //Getting the contents of BackUp table
      ResultSet rs = stmt.executeQuery("SELECT * from BackUp");
      while(rs.next()){
         System.out.println(rs.getInt("Id"));
         System.out.println(rs.getString("Name"));
         System.out.println(rs.getString("Salary"));
         System.out.println(rs.getString("Location"));
         System.out.println(" ");
      }
   }
}

Output

On executing the above program, the following output is generated −

Trigger created
2
Kalyan
40000
Vishakhapatnam
1
Amit
30000
Hyderabad