Derby - Where语句

WHERE子句用于SELECT，DELETE或UPDATE语句中，以指定需要在其上执行操作的行，通常，此子句后面是返回布尔值的条件或表达式，仅对满足给定条件的行执行选择，删除或更新操作。

ij> SELECT * from table_name WHERE condition;
or,
ij> DELETE from table_name WHERE condition;
or,
ij> UPDATE table_name SET column_name=value WHERE condition;

WHERE子句可以使用比较运算符，如=，!= 、、 =以及BETWEEN和LIKE运算符。

Where - 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，具有7条记录，如下所示-

ID |NAME |SALARY |LOCATION
-----------------------------------------------------------------------------
1 |Amit |30000 |Hyderabad
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
4 |Learnfk |15000 |Mumbai
5 |Trupthi |45000 |Kochin
6 |Suchatra |33000 |Pune
7 |Rahul |39000 |Lucknow 

以下SQL DELETE语句获取薪水超过35000的员工的记录-

ij> SELECT * FROM Employees WHERE Salary>35000;

这将产生以下输出-

ID |NAME |SALARY |LOCATION 
---------------------------------------------------
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
5 |Trupthi |45000 |Kochin
7 |Rahul |39000 |Lucknow
4 rows selected

同样，您也可以使用此子句删除和更新记录。

以下示例更新了薪水少于30000的人员的位置。

ij> UPDATE Employees SET Location='Vijayawada' WHERE Salary<35000;
3 rows inserted/updated/deleted

如果您验证表的内容，则可以看到更新后的表，如下所示:

ij> SELECT * FROM Employees;
ID |NAME |SALARY |LOCATION
------------------------------------------------------------------------------
1 |Amit |30000 |Vijayawada
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
4 |Learnfk |15000 |Vijayawada
5 |Trupthi |45000 |Kochin
6 |Suchatra |33000 |Vijayawada
7 |Rahul |39000 |Lucknow
7 rows selected

Where - JDBC示例

以下JDBC示例演示了如何使用WHERE子句并使用JDBC程序在Apache Derby中的表上执行CURD操作，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)。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
public class WhereClauseExample {
   public static void main(String args[]) throws Exception {
      //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
      Statement stmt=conn.createStatement();

      //创建表结构,插入一些测试数据
      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 * FROM Employees WHERE Salary>35000";
      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("Location: "+rs.getString("Location"));
         System.out.println(" ");
      }
   }
}

在执行上述程序时，您将获得以下输出-

无涯教程网

Id: 2
Name: Kalyan
Salary: 43000
Location: Chennai

Id: 3
Name: Renuka
Salary: 50000
Location: Delhi

Id: 5
Name: Trupthi
Salary: 45000
Location: Kochin

Id: 7
Name: Rahul
Salary: 39000
Location: Lucknow