Derby - 查询数据

SELECT语句用于从表中检索数据，这将以称为输出集的表的形式返回数据。

Retrieve Data - 语法

以下是SELECT语句的语法-

ij> SELECT column_name, column_name, ... FROM table_name;
Or,
Ij>SELECT * from table_name

Retrieve Data - 命令行示例

让无涯教程假设无涯教程在数据库中有一个名为Employees的表，如下所示-

ij> CREATE TABLE Employees (
   Id INT NOT NULL GENERATED ALWAYS AS IDENTITY,
   Name VARCHAR(255),
   Salary INT NOT NULL,
   Location VARCHAR(255),
   PRIMARY KEY (Id)
);
> > > > > > > 0 rows inserted/updated/deleted

并且，如下所示插入了四个记录:

ij> INSERT INTO Employees (Name, Salary, Location) VALUES
 ('Amit', 30000, 'Hyderabad'),
 ('Kalyan', 40000, 'Vishakhapatnam'),
 ('Renuka', 50000, 'Delhi'),
 ('Learnfk', 15000, 'Mumbai');
> > > > 4 rows inserted/updated/deleted

以下SQL语句检索表中所有雇员的姓名，年龄和薪水详细信息；

ij> SELECT Id, Name, Salary FROM Employees;

该查询的输出是-

ID |NAME |SALARY
------------------------------------------------------------------------
1 |Amit |30000
2 |Kalyan |40000
3 |Renuka |50000
4 |Learnfk |15000
4 rows selected

如果要一次获取该表的所有记录，请使用*代替列名。

ij> select * from Employees;

这将产生以下输出-

ID |NAME |SALARY |LOCATION
------------------------------------------------------------------
1 |Amit |30000 |Hyderabad
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
4 |Learnfk |15000 |Mumbai
4 rows selected

Retrieve Data - JDBC示例

以下JDBC示例演示了如何使用JDBC程序从Apache Derby中的表中检索数据，在这里，无涯教程使用嵌入式驱动程序连接到名为sampleDB的数据库(如果不存在则创建)。

链接：https://www.learnfk.comhttps://www.learnfk.com/derby/apache-derby-retrieve-data.html

来源：LearnFk无涯教程网

executeQuery()方法返回一个 ResultSet 对象，该对象保存语句的输出，最初，输出集指针将位于第一条记录上，您可以使用ResultSet对象的 next()和 getXXX()方法打印其内容。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class RetrieveData {
   public static void main(String args[]) throws SQLException,
      ClassNotFoundException {
      //注册驱动
      Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
      //创建连接
      String URL="jdbc:derby:sampleDB;create=true";
      Connection conn=DriverManager.getConnection(URL);

      //获取Statement对象
      4Statement stmt=conn.createStatement();

      //创建表结构
      String query="CREATE TABLE Employees("
         + "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "
         + "Name VARCHAR(255), Salary INT NOT NULL, "
         + "Location VARCHAR(255), "
         + "PRIMARY KEY (Id))";
      String query="INSERT INTO Employees("
         + "Name, Salary, Location) VALUES "
         + "('Amit', 30000, 'Hyderabad'), "
         + "('Kalyan', 40000, 'Vishakhapatnam'), "
         + "('Renuka', 50000, 'Delhi'), "
         + "('Learnfk', 15000, 'Mumbai'), "
         + "('Trupthi', 45000, 'Kochin'), "
         + "('Suchatra', 33000, 'Pune'), "
         + "('Rahul', 39000, 'Lucknow'), "
         + "('Trupti', 45000, 'Kochin')";
      //执行SQL语句
      String query="SELECT Id, Name, Salary FROM Employees";
      ResultSet rs=stmt.executeQuery(query);
      while(rs.next()) {
         System.out.println("Id: "+rs.getString("Id"));
         System.out.println("Name: "+rs.getString("Name"));
         System.out.println("Salary: "+rs.getString("Salary"));
         System.out.println(" ");
      }
   }
}

执行上述程序后，您将获得以下输出

无涯教程网

Id: 1
Name: Amit
Salary: 30000

Id: 2
Name: Kalyan
Salary: 43000

Id: 3
Name: Renuka
Salary: 50000

Id: 4
Name: Learnfk
Salary: 15000

Id: 5
Name: Trupthi
Salary: 45000

Id: 6
Name: Suchatra
Salary: 33000

Id: 7
Name: Rahul
Salary: 39000