数据库中alter的用法

　　数据库中alter的用法的用法你知道吗？下面小编就跟你们详细介绍下数据库中alter的用法的用法，希望对你们有用。

　　数据库中alter的用法的用法如下：

　　To add a column to an existing table, the ALTER TABLE syntax is:

　　ALTER TABLE table_name

　　ADD column_name column-definition;

　　For example:

　　ALTER TABLE supplier

　　ADD supplier_name varchar2(50);

　　This will add a column called supplier_name to the supplier table.

　　Syntax #2

　　To add multiple columns to an existing table, the ALTER TABLE syntax is:

　　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) );

　　This will add two columns (supplier_name and city) to the supplier table.

　　Modifying column(s) in a table

　　Syntax #1

　　To modify a column in an existing table, the ALTER TABLE syntax is:

　　ALTER TABLE table_name

　　MODIFY column_name column_type;

　　For example:

　　ALTER TABLE supplier

　　MODIFY supplier_name varchar2(100) not null;

　　This will modify the column called supplier_name to be a data type of varchar2(100) and force the column to not allow null values.

　　Syntax #2

　　To modify multiple columns in an existing table, the ALTER TABLE syntax is:

　　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) );

　　This will modify both the supplier_name and city columns.

　　Drop column(s) in a table

　　Syntax #1

　　To drop a column in an existing table, the ALTER TABLE syntax is:

　　ALTER TABLE table_name

　　DROP COLUMN column_name;

　　For example:

　　ALTER TABLE supplier

　　DROP COLUMN supplier_name;

　　This will drop the column called supplier_name from the table called supplier.

　　Rename column(s) in a table

　　(NEW in Oracle 9i Release 2)

　　Syntax #1

　　Starting in Oracle 9i Release 2, you can now rename a column.

　　To rename a column in an existing table, the ALTER TABLE syntax is:

　　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.