作者：快乐xin_yi | 来源：互联网 | 2018-07-12 07:32
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 |
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ALTER TABLE table_name
RENAME TO new_table_name;
For example:
ALTER TABLE suppliers
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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 |
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ALTER TABLE table_name
ADD column_name column-definition;
For example:
ALTER TABLE supplier
ADD supplier_name varchar2(50);
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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 |
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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 |
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ALTER TABLE table_name
MODIFY column_name column_type;
For example:
ALTER TABLE supplier
MODIFY supplier_name varchar2(100) not null;
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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 |
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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 |
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ALTER TABLE table_name
DROP COLUMN column_name;
For example:
ALTER TABLE supplier
DROP COLUMN supplier_name;
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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 |
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ALTER TABLE table_name
RENAME COLUMN old_name to new_name;
For example:
ALTER TABLE supplier
RENAME COLUMN supplier_name to sname;
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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 |
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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 |
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ALTER TABLE departments
RENAME TO depts;
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Practice Exercise #2:
Based on the employees table below, add a column called salary that is a number(6) datatype.
| 代码如下 |
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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 |
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ALTER TABLE employees
ADD salary number(6);
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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 |
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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 |
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ALTER TABLE customers
ADD ( contact_name varchar2(50),
last_contacted date );
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Practice Exercise #4:
Based on the employees table below, change the employee_name column to a varchar2(75) datatype.
| 代码如下 |
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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 |
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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 |
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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 |
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ALTER TABLE customers
MODIFY ( customer_name varchar2(50) not null,
state varchar2(2) );
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Practice Exercise #6:
Based on the employees table below, drop the salary column.
| 代码如下 |
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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 |
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ALTER TABLE employees
DROP COLUMN salary;
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Practice Exercise #7:
Based on the departments table below, rename the department_name column to dept_name.
| 代码如下 |
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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 |
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ALTER TABLE departments
RENAME COLUMN department_name to dept_name;
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