Altering Tables

Having created tables, you will find there are times that you will want to alter them in some respect. This is done with the alter table command. Some things you might find you need to do with the alter table command include:

* Moving a table to another tablespace

* Adding a column to the table

* Dropping a column from the table

* Changing a column name

* Modify existing constraints or add new constraints

For example, we might want to move the BOOKS table to another tablespace. The alter table command allows us to do this easily:

ALTER TABLE books MOVE TABLESPACE new_users;

You can add a column to a table with the alter table command. In this case, we are adding a column called BOOK_PRICE to the BOOKS table. This column is a number data type:

ALTER TABLE books ADD book_price NUMBER;

You can add multiple columns too as seen in this example where we add a column called BOOK_PUB_NO and BOOK_ALT_TITLE to the BOOKS table:

ALTER TABLE

books

ADD

(

book_pub_no NUMBER,

book_alt_title VARCHAR2(50)

);

It used to be in Oracle that renaming or removing a column from a table was a big deal. You actually had to save the data, drop and re-create the table which can have a horrible impact if it?s a big table in a production environment. Now, you can rename or drop columns from a table with the alter table command.

We demonstrate renaming and dropping a column in the following example. First we rename the BOOK_ISBN column to ISBN_NUMBER. In the second example we drop the BOOK_PUB_NO column:

ALTER TABLE books RENAME COLUMN book_isbn TO isbn_number;

ALTER TABLE books DROP COLUMN book_pub_no;

We discussed primary key constraints earlier in this section. Oracle actually has a number of different constraint types that we will cover in more detail later in this section. However, we should take a moment to see how you can rename a constraint with the alter table command:

ALTER TABLE books

RENAME CONSTRAINT pk_books TO pk_book;

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