Derby - Alert语句

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ALTER TABLE语句允许您更改现有表，使用此您可以执行以下操作-

  • 添加列，添加约束

  • 删除列，删除约束

  • 更改表的行级别锁定

让无涯教程假设已经创建了一个名为Employees的表，如下所示:

ij> CREATE TABLE Employees (
   Id INT NOT NULL GENERATED ALWAYS AS IDENTITY,
   Name VARCHAR(255),
   Salary INT NOT NULL,
   Location VARCHAR(255),
   PRIMARY KEY (Id)
);

并且，使用insert语句将四个记录插入为-

ij> INSERT INTO Employees (Name, Salary, Location) VALUES
 ('Amit', 30000, 'Hyderabad'),
 ('Kalyan', 40000, 'Vishakhapatnam'),
 ('Renuka', 50000, 'Delhi'),
 ('Learnfk', 15000, 'Mumbai');

向表添加列

以下是使用ALTER语句向表中添加列的语法。

ALTER TABLE table_name ADD COLUMN column_name column_type;

使用ALTER语句，无涯教程尝试添加一个名为Age且类型为integer的新列。

无涯教程网

ALTER TABLE Employees ADD COLUMN Age INT;
0 rows inserted/updated/deleted

添加另一列名为Phone_No的整数类型。

ALTER TABLE Employees ADD COLUMN Phone_No BIGINT;
0 rows inserted/updated/deleted

DESCRIBE命令通过列出列及其详细信息来描述指定的表，如果您描述，则表Employees可以观察到新添加的列，如下所示-

ij> DESCRIBE Employees;
COLUMN_NAME |TYPE_NAME|DEC&|NUM&|COLUM&|COLUMN_DEF|CHAR_OCTE&|IS_NULL&
------------------------------------------------------------------------------
ID |INTEGER |0 |10 |10 |AUTOINCRE&|NULL |NO
NAME |VARCHAR |NULL|NULL|255 |NULL |510 |YES
SALARY |INTEGER |0 |10 |10 |NULL |NULL |NO
LOCATION |VARCHAR |NULL|NULL|255 |NULL |510 |YES
AGE |INTEGER |0 |10 |10 |NULL |NULL |YES
PHONE_NO |INTEGER |0 |10 |10 |NULL |NULL |YES
6 rows selected

向表添加约束

以下是使用ALTER语句向表的列添加约束的语法。

ALTER TABLE table_name ADD CONSTRAINT constraint_name constraint (column_name);

其中 constraint 可以是NOT NULL，NULL，PRIMARY KEY，UNIQUE，FOREIGN KEY，CHECK。

使用ALTER语句，无涯教程尝试将约束 UNIQUE 添加到Phone_No列。

链接：https://www.learnfk.comhttps://www.learnfk.com/derby/apache-derby-alter-table-statement.html

来源：LearnFk无涯教程网

ij> ALTER TABLE Employees ADD CONSTRAINT New_Constraint UNIQUE(Phone_No);
0 rows inserted/updated/deleted

一旦您将UNIQUE约束添加到一列，它就不能在两行中具有相同的值，即，每个员工的pady number应该是唯一的。

如果您尝试添加两个具有相同pady number的列，则会出现如下所示的异常。

ij> INSERT INTO Employees (Name, Salary, Location, Age, Phone_No) VALUES
('Amit', 30000, 'Hyderabad', 30, 9848022338);
1 row inserted/updated/deleted
ij> INSERT INTO Employees (Name, Salary, Location, Age, Phone_No) VALUES
('Sumit', 35000, 'Chennai', 25, 9848022338);
ERROR 23505: The statement was aborted because it would have caused a duplicate
key value in a unique or primary key constraint or unique index identified by
'NEW_CONSTRAINT' defined on 'EMPLOYEES'.

从表中删除约束

以下是删除列约束的语法-

ALTER TABLE table_name DROP CONSTRAINT constraint_name;

以下查询删除上面创建的Phone_No列上的约束名称New_Constraint。

ij> ALTER TABLE Employees DROP CONSTRAINT New_Constraint;
0 rows inserted/updated/deleted

由于无涯教程已删除了Phone_No列上的UNIQUE约束，因此您可以添加具有相同pady number的列。

ij> INSERT INTO Employees (Name, Salary, Location, Age, Phone_No) VALUES
('Sumit', 35000, 'Chennai', 25, 9848022338);
1 row inserted/updated/deleted

输入查询 ij > select * from Employees ，如下所示:

ID |NAME |SALARY |LOCATION |AGE |PHONE_NO
-------------------------------------------------------------------------
1 |Amit |30000 |Hyderabad |30 |9848022338
2 |Sumit |35000 |Chennai |25 |9848022338
2 rows selected

从表中删除列

以下是删除列的一列的语法。

ALTER TABLE table_name DROP COLUMN column_name;

以下查询删除名为雇员年龄的列-

ij> ALTER TABLE Employees DROP COLUMN Age;
0 rows inserted/updated/deleted

如果您描述该表，则只能看到4列。

ij> DESCRIBE Employees;
COLUMN_NAME |TYPE_NAME|DEC&|NUM&|COLUM&|COLUMN_DEF|CHAR_OCTE&|IS_NULL&
------------------------------------------------------------------------------
ID |INTEGER |0 |10 |10 |AUTOINCRE&|NULL |NO
NAME |VARCHAR |NULL|NULL|255 |NULL |510 |YES
SALARY |INTEGER |0 |10 |10 |NULL |NULL |NO
LOCATION |VARCHAR |NULL|NULL|255 |NULL |510 |YES
PHONE_NO |BIGINT |0 |10 |19 |NULL |NULL |YES

JDBC示例

以下是使用ALTER查询更改表的JDBC程序-

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class AlterTableExample {
   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 createQuery="CREATE TABLE Employees( "
         + "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "
         + "Name VARCHAR(255), "
         + "Salary INT NOT NULL, "
         + "Location VARCHAR(255), "
         + "PRIMARY KEY (Id))";

      stmt.execute(createQuery);
      System.out.println("Table created");
      System.out.println(" ");

      //执行SQL语句
      String insertQuery="INSERT INTO Employees("
         + "Name, Salary, Location) VALUES "
         + "('Amit', 30000, 'Hyderabad'), "
         + "('Kalyan', 40000, 'Vishakhapatnam'), "
         + "('Renuka', 50000, 'Delhi'), "
         + "('Learnfk', 15000, 'Mumbai'), "
         + "('Trupti', 45000, 'Kochin')";

      stmt.execute(insertQuery);
      System.out.println("Values inserted");
      System.out.println(" ");

      //Executing the query
      String selectQuery="SELECT * FROM Employees";
      ResultSet rs=stmt.executeQuery(selectQuery);
      System.out.println("Contents of the table after inserting the table");
      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(" ");

      //增加表字段与约束
      stmt.execute("ALTER TABLE Employees ADD COLUMN Age INT");
      stmt.execute("ALTER TABLE Employees ADD COLUMN Phone_No BigINT");
      stmt.execute("ALTER TABLE Employees " + "ADD CONSTRAINT New_Constraint UNIQUE(Phone_No)");

      stmt.execute("INSERT INTO Employees "
         + "(Name, Salary, Location, Age, Phone_No) "
         + "VALUES ('Amit', 30000, 'Hyderabad', 30, 9848022338)");
      ResultSet alterResult=stmt.executeQuery("Select * from Employees");
      System.out.println("Contents of the table after altering "
         + "the table and inserting values to it: ");
      while(alterResult.next()) {
         System.out.println("Id: "+alterResult.getString("Id"));
         System.out.println("Name: "+alterResult.getString("Name"));
         System.out.println("Salary: "+alterResult.getString("Salary"));
         System.out.println("Location: "+alterResult.getString("Location"));
         System.out.println("Age: "+alterResult.getString("Age"));
         System.out.println("Phone_No: "+alterResult.getString("Phone_No"));
      }
   }
}

执行上述程序时，将生成以下输出-

Table created

Values inserted

Contents of the table after inserting the table
Id: 1
Name: Amit
Salary: 30000
Location: Hyderabad
Id: 2
Name: Kalyan
Salary: 40000
Location: Vishakhapatnam
Id: 3
Name: Renuka
Salary: 50000
Location: Delhi
Id: 4
Name: Learnfk
Salary: 15000
Location: Mumbai
Id: 5
Name: Trupti
Salary: 45000
Location: Kochin

Contents of the table after altering the table and inserting values to it:
Id: 1
Name: Amit
Salary: 30000
Location: Hyderabad
Age: null
Phone_No: null
Id: 2
Name: Kalyan
Salary: 40000
Location: Vishakhapatnam
Age: null
Phone_No: null
Id: 3
Name: Renuka
Salary: 50000
Location: Delhi
Age: null
Phone_No: null
Id: 4
Name: Learnfk
Salary: 15000
Location: Mumbai
Age: null
Phone_No: null
Id: 5
Name: Trupti
Salary: 45000
Location: Kochin
Age: null
Phone_No: null
Id: 6
Name: Amit
Salary: 30000
Location: Hyderabad
Age: 30
Phone_No: 9848022338

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