数据库|mysql教程
sql alter,alter语句
数据库-mysql教程
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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。
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重命名表
重命名表的基本语法是:
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:
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:
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:
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:
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:
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:
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.
Solution:
The following ALTER TABLE statement would rename the departments table to depts:
Practice Exercise #2:
Based on the employees table below, add a column called salary that is a number(6) datatype.
Solution:
The following ALTER TABLE statement would add a salary column to the employees table:
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.
Solution:
The following ALTER TABLE statement would add the contact_name and last_contacted columns to the customers table:
Practice Exercise #4:
Based on the employees table below, change the employee_name column to a varchar2(75) datatype.
Solution:
The following ALTER TABLE statement would change the datatype for the employee_name column to 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.
Solution:
The following ALTER TABLE statement would modify the customer_name and state columns accordingly in the customers table:
Practice Exercise #6:
Based on the employees table below, drop the salary column.
Solution:
The following ALTER TABLE statement would drop the salary column from the employees table:
Practice Exercise #7:
Based on the departments table below, rename the department_name column to dept_name.
解决方案:
下面的ALTER TABLE语句将重新命名department_name列dept_name部门表: