Apache Derby 简明教程
Apache Derby - Triggers
在数据库中，触发器是每当发生事件时执行的语句/代码。一旦您为表上的特定事件创建了触发器，触发器中指定的代码将每次发生事件时执行。您可以在单个表上创建多个触发器。
本章将向您介绍如何使用 Apache Derby 创建和删除触发器。
Creating a trigger
您可以使用 CREATE TRIGGER 语句在 Derby 中创建触发器。
Syntax
以下是 CREATE TRIGGER 查询的语法。
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]
StatementExample
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)
);并在其中插入了 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，我们的目的是存储 Emp 表中的已删除行。
CREATE TABLE BackUp (
Id INT NOT NULL,
Name VARCHAR(255),
Salary INT NOT NULL,
Location VARCHAR(255)
);以下查询在 DELETE 查询表中创建了一个名为 Emp 的触发器。它将 Emp 的已删除行存储在 Backup 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，如下所示：
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，你可以在其中观察已删除的行。
ij> select * from BackUp;
ID |NAME |SALARY |LOCATION
-------------------------------------------------------------------------
2 |Kalyan |40000 |Vishakhapatnam
1 |Amit |30000 |Hyderabad
2 rows selectedJDBC example
以下 JDBC 程序在 Derby 中创建和删除触发器。
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(" ");
}
}
}