在创建表时，在某些情况下，我们需要自动生成/增加列的值（例如ID）。各种数据库以不同的方式支持此功能。
在MySQL数据库中，您可以使用以下语法声明列自动递增。
CREATE TABLE table_name( ID INT PRIMARY KEY AUTO_INCREMENT, column_name1 data_type1, column_name2 data_type2, column_name3 data_type3, column_name4 data_type4, ............ ........... );
在表中插入记录时，无需在自动增加的列下插入值。这些将自动生成。
默认情况下，自动增加的列的初始值为1。您可以使用ALTER TABLE查询来更改它，如下所示-
alter table table_name AUTO_INCREMENT = 1001
让我们使用CREATE语句在MySQL数据库中创建一个名称为sales的表，其中一列自动递增，如下所示-
CREATE TABLE Sales( ID INT PRIMARY KEY AUTO_INCREMENT, ProductName VARCHAR (20), CustomerName VARCHAR (20), DispatchDate date, DeliveryTime time, Price INT, Location VARCHAR(20) );
以下JDBC程序将auto-incremented列的初始值设置为1001，并在其中插入6条记录。
import java.sql.Connection;
import java.sql.Date;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.Time;
public class SettingInitialValue_AutoIncrement_Pstmt {
public static void main(String args[]) throws SQLException {
//注册驱动程序
DriverManager.registerDriver(new com.mysql.jdbc.Driver());
//获得连接
String mysqlUrl = "jdbc:mysql://localhost/sample_database";
Connection con = DriverManager.getConnection(mysqlUrl, "root", "password");
System.out.println("Connection established......");
//设置自动递增列的初始值
Statement stmt = con.createStatement();
stmt.execute("alter table Sales AUTO_INCREMENT = 1001");
//查询以将值插入销售表
String insertQuery = "INSERT INTO Sales (ProductName, CustomerName, DispatchDate, DeliveryTime, Price, Location) VALUES (?, ?, ?, ?, ?, ?)";
//创建一个PreparedStatement对象
PreparedStatement pstmt = con.prepareStatement(insertQuery,Statement.RETURN_GENERATED_KEYS);
pstmt.setString(1, "Key-Board");
pstmt.setString(2, "Raja");
pstmt.setDate(3, new Date(1567315800000L));
pstmt.setTime(4, new Time(1567315800000L));
pstmt.setInt(5, 7000);
pstmt.setString(6, "Hyderabad");
pstmt.addBatch();
pstmt.setString(1, "Earphones");
pstmt.setString(2, "Roja");
pstmt.setDate(3, new Date(1556688600000L));
pstmt.setTime(4, new Time(1556688600000L));
pstmt.setInt(5, 2000);
pstmt.setString(6, "Vishakhapatnam");
pstmt.addBatch();
pstmt.setString(1, "Mouse");
pstmt.setString(2, "Puja");
pstmt.setDate(3, new Date(1551418199000L));
pstmt.setTime(4, new Time(1551418199000L));
pstmt.setInt(5, 3000);
pstmt.setString(6, "Vijayawada");
pstmt.addBatch();
pstmt.setString(1, "Mobile");
pstmt.setString(2, "Vanaja");
pstmt.setDate(3, new Date(1551415252000L));
pstmt.setTime(4, new Time(1551415252000L));
pstmt.setInt(5, 9000);
pstmt.setString(6, "Chennai");
pstmt.addBatch();
pstmt.setString(1, "Headset");
pstmt.setString(2, "Jalaja");
pstmt.setDate(3, new Date(1554529139000L));
pstmt.setTime(4, new Time(1554529139000L));
pstmt.setInt(5, 6000);
pstmt.setString(6, "Goa");
pstmt.addBatch();
System.out.println("Records inserted......");
//执行批处理
pstmt.executeBatch();
}
}输出结果
Connection established...... Records inserted......
如果使用SELECT语句验证Sales表的内容，则可以看到ID值从1001开始的插入记录，如下所示-
mysql> select * from sales; +------+-------------+--------------+--------------+--------------+-------+----------------+ | ID | ProductName | CustomerName | DispatchDate | DeliveryTime | Price | Location | +------+-------------+--------------+--------------+--------------+-------+----------------+ | 1001 | Key-Board | Raja | 2019-09-01 | 11:00:00 | 7000 | Hyderabad | | 1002 | Earphones | Roja | 2019-05-01 | 11:00:00 | 2000 | Vishakhapatnam | | 1003 | Mouse | Puja | 2019-03-01 | 10:59:59 | 3000 | Vijayawada | | 1004 | Mobile | Vanaja | 2019-03-01 | 10:10:52 | 9000 | Chennai | | 1005 | Headset | Jalaja | 2019-04-06 | 11:08:59 | 6000 | Goa | +------+-------------+--------------+--------------+--------------+-------+----------------+ 5 rows in set (0.00 sec)