HSQLDB Alter Command

The HSQLDB (HyperSQL Database) Alter command is used to alter the structure of tables and other database objects in an HSQLDB database. It allows programmers to modify existing database objects to meet the changing requirements of their applications.

Syntax

The syntax for the HSQLDB Alter command is as follows:

ALTER TABLE table_name [ * | column_name datatype [ DEFAULT initial_value ] [Column Constraint...]]
       | ALTER TABLE ADD CONSTRAINT constraint_name constraint_definition
       | ALTER TABLE DROP CONSTRAINT constraint_name

Description

• ALTER TABLE: This clause is used to modify the structure of a table.

• table_name: Specifies the name of the table to be altered.

• * | column_name datatype [ DEFAULT initial_value ] [Column Constraint...]: Specifies the column(s) to be altered in the table. The * is used to add a new column at the end of the table, while column_name is used to alter an existing column.

  • datatype: Specifies the new data type for the column.

  • DEFAULT initial_value: Optional. Specifies a default value for the column.

  • Column Constraint...: Optional. Specifies additional constraints for the column, such as NOT NULL, UNIQUE, etc.

• ADD CONSTRAINT: This clause is used to add a new constraint to a table.

• constraint_name: Specifies the name of the constraint to be added.

• constraint_definition: Specifies the definition of the constraint, including the type of constraint (e.g., PRIMARY KEY, FOREIGN KEY, CHECK), and the columns involved.

• DROP CONSTRAINT: This clause is used to drop an existing constraint from a table.

• constraint_name: Specifies the name of the constraint to be dropped.

Example

Altering a Table's Column

To modify the structure of a table by altering a column, you can use the following syntax:

ALTER TABLE employees ALTER COLUMN salary DECIMAL(10,2) DEFAULT 0.00;

This example alters the column 'salary' in the 'employees' table. It changes the data type to DECIMAL with a precision of 10 and scale of 2, and sets a default value of 0.00 for the column.

Adding a Constraint

To add a constraint to a table, you can use the following syntax:

ALTER TABLE employees ADD CONSTRAINT pk_employees PRIMARY KEY (employee_id);

This example adds a primary key constraint named 'pk_employees' to the 'employees' table, with the 'employee_id' column as the primary key.

Dropping a Constraint

To drop an existing constraint from a table, you can use the following syntax:

ALTER TABLE employees DROP CONSTRAINT pk_employees;

This example drops the primary key constraint named 'pk_employees' from the 'employees' table.

Conclusion

The HSQLDB Alter command is an essential tool for programmers working with the HSQLDB database. It allows for the modification of table structure and the addition or removal of constraints, providing the flexibility to adapt the database to changing application requirements.