Oracle Alter Table

The ALTER TABLE statement in Oracle is used to modify the structure of an existing table in a database. It allows programmers to add, modify, or delete columns, constraints, and other table attributes.

Syntax

The basic syntax for altering a table in Oracle is as follows:

ALTER TABLE table_name
  tabe_action;

Here, table_name is the name of the table you want to modify, and table_action is the specific alteration you want to perform on the table.

Alterations Supported by Oracle

1. Add Columns

To add new columns to an existing table, you can use the ADD keyword:

ALTER TABLE table_name
  ADD (column_name1 datatype1 [DEFAULT expression],
       column_name2 datatype2 [DEFAULT expression],
       ...);

Each column is specified with a name and a data type. Additionally, you can specify a default value for the column using the DEFAULT keyword followed by an expression.

2. Modify Columns

If you need to modify the data type or size of a column, you can use the MODIFY keyword:

ALTER TABLE table_name
  MODIFY column_name datatype [DEFAULT expression];

This statement allows you to change the data type and optionally assign a default value for the column.

3. Rename Columns

To rename an existing column in a table, you can use the RENAME COLUMN keyword:

ALTER TABLE table_name
  RENAME COLUMN old_column_name TO new_column_name;

This allows you to change the name of a specific column in a table.

4. Drop Columns

To remove one or more columns from a table, you can use the DROP COLUMN keyword:

ALTER TABLE table_name
  DROP COLUMN column_name;

You can specify multiple columns to be dropped by separating them with commas.

5. Add Constraints

To add constraints to an existing table, such as primary key, unique, or foreign key constraints, you can use the ADD CONSTRAINT keyword:

ALTER TABLE table_name
  ADD CONSTRAINT constraint_name constraint_definition;

Here, constraint_name is the name you assign to the constraint, and constraint_definition is the specific constraint you want to add.

6. Modify Constraints

If you need to modify an existing constraint, such as changing the constraint condition or disabling/enabling it, you can use the MODIFY CONSTRAINT keyword:

ALTER TABLE table_name
  MODIFY CONSTRAINT constraint_name constraint_definition;

This statement allows you to alter the definition of an existing constraint. Note that only certain constraint types can be modified in Oracle.

7. Drop Constraints

To remove constraints from an existing table, you can use the DROP CONSTRAINT keyword:

ALTER TABLE table_name
  DROP CONSTRAINT constraint_name;

This statement allows you to drop a specific constraint from a table.

Conclusion

The ALTER TABLE statement in Oracle provides various operations to modify the structure of existing tables. It allows you to add, modify, or drop columns, as well as add, modify, or drop constraints. Understanding how to use ALTER TABLE is essential for effectively managing database schema changes.