5.5. Modifying Tables

5.5. Modifying Tables

5.5.修改表

When you create a table and you realize that you made a mistake, or the requirements of the application  change, you can drop the table and create it again. But this is not a convenient option if the table is  already filled with data, or if the table is referenced by other database objects (for instance a foreign key  constraint). Therefore PostgreSQL provides a family of commands to make modifications to existing  tables. Note that this is conceptually distinct from altering the data contained in the table: here we are  interested in altering the definition, or structure, of the table.

当你创建了一张错误的表，或者应用程序的需求改变了，你可以删掉该表，并重建它。但是当表中有数据或者该表被其他表引用（例如，外键索引）的时候，这样操作并不太方便。因此，PostgreSQL提供了一系列命令以修改表。请注意，从概念上讲，这与改变表中的数据不同：此处，我们的关注点在于修改表的定义或结构。

 

You can:

• Add columns

• Remove columns

• Add constraints

• Remove constraints

• Change default values

• Change column data types

• Rename columns

• Rename tables

All these actions are performed using the ALTER TABLE command, whose reference page contains  details beyond those given here.

你可以：

• 添加列
• 删除列
• 添加约束
• 删除约束
• 改变默认值
• 改变列的数据类型
• 重命名列
• 重命名表

以上所有的操作均通过 ALTER TABLE命令实现，此命令的参考页提供了比本章更详细的相关信息。