WHERE子句用于SELECT,DELETE或UPDATE语句,以指定需要在其上执行操作的行.通常,此子句后跟一个返回布尔值的条件或表达式,只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,例如=,!=,
假设我们在数据库中有一个名为Employees的表,其中包含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 |Archana |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.
以下示例更新薪水低于30000的人的位置.
ij> UPDATE Employees SET Location = 'Vijayawada' WHERE Salary<35000; 3 rows inserted/updated/deleted
如果验证表的内容,可以看到更新的表,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 |Archana |15000 |Vijayawada 5 |Trupthi |45000 |Kochin 6 |Suchatra |33000 |Vijayawada 7 |Rahul |39000 |Lucknow 7 rows selected
本节教你如何使用WHERE子句并使用JDBC应用程序对Apache Derby数据库中的表执行CURD操作.
如果要请求使用网络客户端的Derby网络服务器,确保服务器已启动并正在运行.网络客户端驱动程序的类名是org.apache.derby.jdbc.ClientDriver,URL是jdbc:derby://localhost:1527/ DATABASE_NAME ; create = true; user = USER_NAME; passw
ord = PASSWORD ".
按照以下步骤使用WHERE子句并对表执行CURD操作在Apache Derby中
要与数据库通信,首先需要注册驱动程序. b> forName()类的方法类接受表示类名的String值将其加载到内存中,自动注册它.使用此方法注册驱动程序
通常,我们与数据库进行通信的第一步是连接它.连接 class表示与数据库服务器的物理连接.您可以通过调用 DriverManager的 getConnection()方法来创建连接对象. b>上课.使用此方法创建连接.
您需要创建语句或 PreparedStatement 或 CallableStatement 对象,用于将SQL语句发送到数据库.您可以分别使用方法 createStatement(),prepareStatement()和prepareCall()来创建它们.使用适当的方法创建其中任何一个对象.
创建语句后,需要执行它. Statement 类提供了各种方法来执行查询,如 execute()方法,以执行返回多个结果集的语句. executeUpdate()方法执行INSERT,UPDATE,DELETE等查询. executeQuery()方法返回数据的结果.使用这些方法之一并执行先前创建的语句.
以下JDBC示例演示如何使用WHERE子句并对a执行CURD操作Apache Derby中的表使用JDBC程序.在这里,我们使用嵌入式驱动程序连接到名为sampleDB的数据库(如果它不存在将创建).
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 { //Registering the driver Class.forName("org.apache.derby.jdbc.EmbeddedDriver"); //Getting the Connection object String URL = "jdbc:derby:sampleDB;create=true"; Connection conn = DriverManager.getConnection(URL); //Creating the Statement object Statement stmt = conn.createStatement(); //Creating a table and populating it 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'), " + "('Archana', 15000, 'Mumbai'), " + "('Trupthi', 45000, 'Kochin'), " + "('Suchatra', 33000, 'Pune'), " + "('Rahul', 39000, 'Lucknow'), " + "('Trupti', 45000, 'Kochin')"; //Executing the query 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(" "); } } }
执行上述程序时,你将获得以下输出 :
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