在数据库中，触发器是在事件发生时执行的语句/代码.为表上的特定事件创建触发器后，每次事件发生时都会执行触发器中指定的代码.您可以在一个表上创建多个触发器.
本章教您如何使用Apache Derby创建和删除触发器.
您可以使用CREATE TRIGGER语句在Derby中创建触发器.
以下是语法CREATE TRIGGER查询.
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假设我们在Derby中创建了一个名为Emp的表，如下所示.
CREATE TABLE Emp ( Id INT NOT NULL, Name VARCHAR(255), Salary INT NOT NULL, Location VARCHAR(255) );
并在其中插入5行.
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');
如果我们有另一个名为BackUp的表，我们打算在此处存储Emp表中已删除的行.
CREATE TABLE BackUp ( Id INT NOT NULL, Name VARCHAR(255), Salary INT NOT NULL, Location VARCHAR(255) );
以下查询在名为 Emp 的DELETE查询表上创建触发器.它将已删除的 Emp 行存储到备份表中.
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);
现在，从Emp表中删除一行 : 去;
ij> Delete From Emp where Name = 'Kalyan'; 1 row inserted/updated/deleted ij> Delete From Emp where Name = 'Amit'; 1 row inserted/updated/deleted
如果验证BackUp表，则可以观察其中已删除的行.
ij>从BackUp中选择*; ID | NAME | SALARY | LOCATION ----------------------------------- -------------------------------------- 2 | Kalyan | 40000 | Vishakhapatnam 1 | Amit | 30000 | Hyderabad 选择2行
您可以使用DROP TRIGGER语句删除Derby中的触发器.
以下是DROP TRIGGER查询的语法 :
ij> Drop trigger tigger_name;
以下示例删除上面创建的触发器my_trigger :
ij> Drop trigger my_trigger; 0 rows inserted/updated/deleted
以下JDBC程序创建和删除触发器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 \r\n"
+"(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(" ");
}
}
}执行上述程序时，生成以下输出 :
Trigger created 2 Kalyan 40000 Vishakhapatnam 1 Amit 30000 Hyderabad
