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📜  Apache Derby-触发器

📅  最后修改于: 2020-11-28 12:55:38             🧑  作者: Mango

在数据库中，触发器是每当事件发生时都会执行的语句/代码。为表上的特定事件创建触发器后，每次事件发生时，触发器中指定的代码都会执行。您可以在单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。

创建触发器

您可以使用CREATE TRIGGER语句在Derby中创建触发器。

句法

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]
Statement

例

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，而我们的目的是在其中存储从Emp表删除的行。

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Emp的已删除行存储到表Backup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-

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

如果您验证了备份表，则可以观察其中的已删除行。

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

2 rows selected

删除触发器

您可以使用DROP TRIGGER语句在Derby中删除触发器。

句法

以下是DROP TRIGGER查询的语法-

ij> Drop trigger tigger_name;

例

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

JDBC示例

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(" ");
      }
   }
}

输出

执行上述程序时，将生成以下输出-

Trigger created
2
Kalyan
40000
Vishakhapatnam
1
Amit
30000
Hyderabad