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Java ResultSet wasNull（）方法与示例
当我们执行某些SQL查询（通常是SELECT查询）时，它们返回表格数据。
该java.sql.ResultSet中的接口表示由SQL语句返回这样的表格数据。
即ResultSet 对象保存由执行查询数据库的语句的方法返回的表格数据（通常为Statement接口的executeQuery（）方法）。
ResultSet对象具有指向当前行的光标/指针。最初，此光标位于第一行之前。
ResultSet接口的wasNull（）方法确定最后读取的列是否具有Null值。即，每当您使用getter方法（getInt（），getString等...）读取ResultSet的列的内容时，都可以使用wasNull（）方法确定其（列）是否包含空值。
此方法返回一个布尔值，该值指定刚读取的列是否包含空值。如果列中有空值，则此方法返回true，否则返回false。
让我们使用CREATE语句在MySQL数据库中创建一个名为MyPlayers的表，如下所示-
CREATE TABLE MyPlayers(
ID INT,
First_Name VARCHAR(255),
Last_Name VARCHAR(255),
Date_Of_Birth date,
Place_Of_Birth VARCHAR(255),
Country VARCHAR(255),
PRIMARY KEY (ID)
);
现在，我们将使用INSERT语句在MyPlayers表中插入7条记录-
insert into MyPlayers values(1, 'Shikhar', 'Dhawan', DATE('1981-12-05'), 'Delhi', 'India');
insert into MyPlayers values(2, 'Jonathan', 'Trott', DATE('1981-04-22'), 'CapeTown', 'SouthAfrica');
insert into MyPlayers values(3, 'Kumara', 'Sangakkara', DATE('1977-10-27'), 'Matale', 'Srilanka');
insert into MyPlayers values(4, 'Virat', 'Kohli', DATE('1988-11-05'), 'Delhi', 'India');
insert into MyPlayers values(5, 'Rohit', 'Sharma', DATE('1987-04-30'), 'Nagpur', 'India');
insert into MyPlayers values(6, 'Ravindra', 'Jadeja', DATE('1988-12-06'), 'Nagpur', 'India');
insert into MyPlayers values(7, 'James', 'Anderson', DATE('1982-06-30'), 'Burnley', 'England');
接下来的JDBC程序建立与数据库的连接，将具有空值的新记录插入MyPlayers表中，将其内容检索到ResultSet对象中，并使用wasNull（）方法打印包含空值的记录的内容。
例
import java.sql.Connection;
import java.sql.Date;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class ResultSet_wasNull {
public static void main(String args[]) throws SQLException {
//Registering the Driver
DriverManager.registerDriver(new com.mysql.jdbc.Driver());
//Getting the connection
String mysqlUrl = "jdbc:mysql://localhost/mydatabase";
Connection con = DriverManager.getConnection(mysqlUrl, "root", "password");
System.out.println("Connection established......");
//Creating the Statement
Statement stmt = con.createStatement();
//Insert a row into the MyPlayers table
PreparedStatement pstmt = con.prepareStatement("INSERT INTO MyPlayers values (?, ?, ?, ?, ?, ? )");
pstmt.setInt(1, 8);
pstmt.setString(2, "Ryan");
pstmt.setString(3, "McLaren");
pstmt.setDate(4, new Date(413596800000L));
pstmt.setString(5, "Kumberly");
pstmt.setNull(6, java.sql.Types.VARCHAR);
pstmt.executeUpdate();
//Query to retrieve records
String query = "Select * from MyPlayers";
//Executing the query
ResultSet rs = stmt.executeQuery(query);
//Verifying whether last row read was null
while(rs.next()) {
int id = rs.getInt("ID");
String first_name = rs.getString("First_Name");
String last_name = rs.getString("Last_Name");
Date date_of_birth = rs.getDate("Date_Of_Birth");
String place_of_birth = rs.getString("Place_Of_Birth");
String country = rs.getString("Country");
boolean bool = rs.wasNull();
if(bool) {
System.out.print("Id: "+id+", ");
System.out.print("First Name: "+first_name+", ");
System.out.print("Last Name: "+last_name+", ");
System.out.print("Date Of Birth: "+date_of_birth+", ");
System.out.print("Place Of Birth: "+place_of_birth+", ");
System.out.print("Country: "+country);
System.out.println(" ");
}
}
}
}
输出
Connection established......
Id: 8, First Name: Ryan, Last Name: McLaren, Date Of Birth: 1983-02-09, Place Of Birth: Kumberly, Country: null
在上面例子中，会判断Country列是否为SQL NULL。所以，wasNULL()方法会判断最后一个接收到的最后一个值是否为空