如何使用JDBC API将主键约束添加到数据库中表的列?
您可以使用ALTER TABLE命令将主键约束添加到表的列。
语法
ALTER TABLE table_nameADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);
假设我们在数据库中有一个名为Dispatches的表,其中有7列,分别是id,CustomerName,DispatchDate,DeliveryTime,Price和Location,其描述如下所示:
+--------------+--------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| ProductName | varchar(255) | YES | | NULL | |
| CustomerName | varchar(255) | YES | | NULL | |
| DispatchDate | date | YES | | NULL | |
| DeliveryTime | time | YES | | NULL | |
| Price | int(11) | YES | | NULL | |
| Location | text | YES | | NULL | |
| ID | int(11) | NO | | NULL | |
+--------------+--------------+------+-----+---------+-------+
以下JDBC程序建立与MySQL数据库的连接,并向名为id的列添加主键约束。
import java.sql.Connection;import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
public class Adding_PrimaryKey_Constraint {
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/mydatabase";
Connection con = DriverManager.getConnection(mysqlUrl, "root", "password");
System.out.println("Connection established......");
//创建语句
Statement stmt = con.createStatement();
//查询更改表
String query = "ALTER TABLE Sales ADD CONSTRAINT MyPrimaryKey PRIMARY KEY(ID)";
//执行查询
stmt.executeUpdate(query);
System.out.println("Constraint added......");
}
}
输出结果
Connection established......Constraint added......
由于我们在名为id列的列上添加了主要约束,因此,如果使用describe命令获得Sales表的描述,则可以观察到键值PRI与Id相反添加。
mysql> describe sales;+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| ProductName | varchar(255) | YES | | NULL | |
| CustomerName | varchar(255) | YES | | NULL | |
| DispatchDate | date | YES | | NULL | |
| DeliveryTime | time | YES | | NULL | |
| Price | int(11) | YES | | NULL | |
| Location | text | YES | | NULL | |
| ID | int(11) | NO | PRI | NULL | |
+--------------+--------------+------+-----+---------+-------+
7 rows in set (0.00 sec)
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