Apache Derby 简明教程

Apache Derby - Alter Table Statement

ALTER TABLE 语句允许你更改现有表。使用它可以执行以下操作 −

  1. 添加列，添加约束

  2. 删除列，删除约束

  3. 更改表的行级锁定

让我们假设我们已创建一个名为 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)
);

而且，使用 insert 语句插入四条记录，如下 −

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

Adding a column to a Table

以下是使用 ALTER 语句将列添加到表中的语法。

ALTER TABLE table_name ADD COLUMN column_name column_type;

Example

使用 ALTER 语句，我们尝试使用类型 integer 添加一个名为 Age 的新列。

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

使用 Gemini 将此文本翻译成中文:

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

在类型为整型的列中添加另一个名为 Phone_No 的列。

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
AGE         |INTEGER  |0   |10   |10   |NULL      |NULL      |YES
PHONE_NO    |INTEGER  |0   |10   |10   |NULL      |NULL      |YES
6 rows selected

Adding a constraint to a table

DESCRIBE 命令通过列出列とその詳細信息来描述指定表格（如果表存在）。如果您对 Employees 表格执行 DESCRIBE，您可以观察到新添加的列，如下所示：

ALTER TABLE table_name ADD CONSTRAINT constraint_name constraint (column_name);

以下是使用 ALTER 语句向表格列添加约束的语法：

Example

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

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

使用 ALTER 语句，我们尝试向 Phone_No 列添加约束 UNIQUE 。

一旦您向列添加 UNIQUE 约束，则两个行不能具有相同的值，即每个员工的电话号码应唯一。

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'.

Dropping a constraint from a table

如果您尝试添加两个具有相同电话号码的列，则会收到异常，如下所示：

ALTER TABLE table_name DROP CONSTRAINT constraint_name;

Example

以下是删除列约束的语法：

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

以下查询在上面创建的列 Phone_No 中删除了约束名称 New_Constraint。

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

由于我们已删除列 Phone_No 上的 UNIQUE 约束，因此您可以添加具有相同电话号码的列。

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

Dropping a column from a table

您可以通过以下方式验证表格的内容：ij> select * from Employees

ALTER TABLE table_name DROP COLUMN column_name;

Example

以下是删除列的语法的列。

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

以下查询删除名为 age of the employee 的列：

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

Altering table using JDBC program

如果您描述表格，您只能看到 4 列。

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 {
      //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();

      //Executing the query
      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(" ");

      //Executing the query
      String insertQuery = "INSERT INTO Employees("
         + "Name, Salary, Location) VALUES "
         + "('Amit', 30000, 'Hyderabad'), "
         + "('Kalyan', 40000, 'Vishakhapatnam'), "
         + "('Renuka', 50000, 'Delhi'), "
         + "('Archana', 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(" ");

      //Altering the table
      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"));
      }
   }
}

Output

以下是使用 ALTER 查询来更改表格的 JDBC 程序：

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: Archana
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: Archana
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