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.