sql alter table语句用法

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ALTER TABLE语句允许您重命名一个现有的表。它也可以被用于从现有的表添加,修改或删除一列,下面我们来看看alter语句的多种用法吧。

重命名表
重命名表的基本语法是:

 代码如下 复制代码

ALTER TABLE table_name
 RENAME TO new_table_name;

For example:

ALTER TABLE suppliers

这将重命名的供应商表供应商。

 

表中添加列(S)
语法#1

要添加到现有的表列,ALTER TABLE的语法是:

 代码如下 复制代码

ALTER TABLE table_name
 ADD column_name column-definition;

For example:

ALTER TABLE supplier
 ADD supplier_name  varchar2(50);

这会增加供应商表中的列称为supplier_name。

 

语法#2

要添加到现有表的多个列,ALTER TABLE的语法是:

 代码如下 复制代码

ALTER TABLE table_name
ADD ( column_1 column-definition,
  column_2 column-definition,
  ... 
  column_n column_definition );

For example:

ALTER TABLE supplier
ADD ( supplier_name varchar2(50),
  city varchar2(45) );

这将增加两列(supplier_name市)的供应商表。

 

修改表中的列(S)
语法#1

要修改现有表列,ALTER TABLE的语法是:

 代码如下 复制代码

ALTER TABLE table_name
 MODIFY column_name column_type;

For example:

ALTER TABLE supplier
 MODIFY supplier_name   varchar2(100)     not null;


这将修改所谓supplier_name,是一个VARCHAR2数据类型(100),并迫使列不允许空值的列。

 

语法#2

要修改现有表中的多个列,ALTER TABLE的语法是:

 代码如下 复制代码

ALTER TABLE table_name
MODIFY ( column_1 column_type,
  column_2 column_type,
  ... 
  column_n column_type );

For example:

ALTER TABLE supplier
MODIFY ( supplier_name varchar2(100) not null,
 city varchar2(75)   );

这将修改supplier_name和城市列。

 

(S)在一个表中删除列
语法#1

要删除一个现有的表列,ALTER TABLE的语法是:

 代码如下 复制代码

ALTER TABLE table_name
 DROP COLUMN column_name;

For example:

ALTER TABLE supplier
 DROP COLUMN supplier_name;

这将下降supplier_name名为供应商的表列。

 

在表重命名列(S)
(新在Oracle 9i第2版)
语法#1

在Oracle9i第2版开始,你现在可以重命名列。

要在现有的表重命名列,ALTER TABLE的语法是:

 代码如下 复制代码

ALTER TABLE table_name
 RENAME COLUMN old_name to new_name;

For example:

ALTER TABLE supplier
 RENAME COLUMN supplier_name to sname;


This will rename the column called supplier_name to sname.

Acknowledgements: Thanks to Dave M., Craig A., and Susan W. for contributing to this solution!

 

Practice Exercise #1:

Based on the departments table below, rename the departments table to depts.

 代码如下 复制代码
CREATE TABLE departments
( department_id number(10) not null,
  department_name varchar2(50) not null,
  CONSTRAINT departments_pk PRIMARY KEY (department_id)
);   

 

Solution:

The following ALTER TABLE statement would rename the departments table to depts:

 代码如下 复制代码

ALTER TABLE departments
 RENAME TO depts;

Practice Exercise #2:

Based on the employees table below, add a column called salary that is a number(6) datatype.

 代码如下 复制代码

CREATE TABLE employees
( employee_number number(10) not null,
  employee_name varchar2(50) not null,
  department_id number(10), 
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);   

Solution:

The following ALTER TABLE statement would add a salary column to the employees table:

 代码如下 复制代码

ALTER TABLE employees
 ADD salary number(6);

Practice Exercise #3:

Based on the customers table below, add two columns - one column called contact_name that is a varchar2(50) datatype and one column called last_contacted that is a date datatype.

 代码如下 复制代码

CREATE TABLE customers
( customer_id number(10) not null,
  customer_name varchar2(50) not null,
  address varchar2(50), 
  city varchar2(50), 
  state varchar2(25), 
  zip_code varchar2(10), 
  CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);   

Solution:

The following ALTER TABLE statement would add the contact_name and last_contacted columns to the customers table:

 代码如下 复制代码

ALTER TABLE customers
ADD ( contact_name varchar2(50),
  last_contacted date );

Practice Exercise #4:

Based on the employees table below, change the employee_name column to a varchar2(75) datatype.

 代码如下 复制代码
CREATE TABLE employees
( employee_number number(10) not null,
  employee_name varchar2(50) not null,
  department_id number(10), 
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);   

 

Solution:

The following ALTER TABLE statement would change the datatype for the employee_name column to varchar2(75):

 代码如下 复制代码

ALTER TABLE employees
 MODIFY employee_name varchar2(75);

Practice Exercise #5:

Based on the customers table below, change the customer_name column to NOT allow null values and change the state column to a varchar2(2) datatype.

 代码如下 复制代码

CREATE TABLE customers
( customer_id number(10) not null,
  customer_name varchar2(50),  
  address varchar2(50), 
  city varchar2(50), 
  state varchar2(25), 
  zip_code varchar2(10), 
  CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);   

Solution:

The following ALTER TABLE statement would modify the customer_name and state columns accordingly in the customers table:

 代码如下 复制代码

ALTER TABLE customers
MODIFY ( customer_name varchar2(50) not null,
  state varchar2(2) );

Practice Exercise #6:

Based on the employees table below, drop the salary column.

 代码如下 复制代码

CREATE TABLE employees
( employee_number number(10) not null,
  employee_name varchar2(50) not null,
  department_id number(10), 
  salary number(6), 
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);   

Solution:

The following ALTER TABLE statement would drop the salary column from the employees table:

 代码如下 复制代码

ALTER TABLE employees
 DROP COLUMN salary;

Practice Exercise #7:

Based on the departments table below, rename the department_name column to dept_name.

 代码如下 复制代码

CREATE TABLE departments
( department_id number(10) not null,
  department_name varchar2(50) not null,
  CONSTRAINT departments_pk PRIMARY KEY (department_id)
);   

解决方案:

下面的ALTER TABLE语句将重新命名department_name列dept_name部门表:

 代码如下 复制代码

ALTER TABLE departments
 RENAME COLUMN department_name to dept_name;