Delete Constraints
To delete a constraint using SQL, use the ALTER TABLE...DROP CONSTRAINT statement. When you drop a constraint, you must specify RESTRICT or CASCADE. RESTRICT aborts the operation if there are any constraints that depend on the constraint being dropped. CASCADE deletes all dependent constraints.
In VDBA, the Create Table dialog allows you to delete any constraint as you are designing your table, without restrictions. After you have saved the table, remove constraints using the Alter Table dialog.
Because removing constraints is more complicated when altering a table, each dialog in VDBA that allows you to work with constraints gives you two delete options when altering a table. These same dialogs give you only one delete option when creating a table:
• Delete performs a restrictive delete, assuming that there are no dependent constraints. If you delete a constraint using this button, the constraint is dropped only if there are no dependent constraints; otherwise, if there are dependent constraints, the delete operation results in an error.
• Del Cascade performs a cascading delete by also dropping all dependent constraints. This is not available--and not needed--when creating a table.
For example, a unique constraint in one table can have a dependent referential constraint in another table. In this case, if you altered the table in which the unique constraint was defined and attempted to perform a Delete operation in the Table Level Unique Constraint dialog, it results in an error due to the existence of the dependent referential constraint. To delete the unique constraint successfully, use Del Cascade, which also deletes the referential constraint in the other table.
Note: In VDBA, column-level unique constraints are defined directly in the Create Table or Alter Table dialog. You cannot, however, remove a column-level unique constraint simply by disabling its Unique check box in the Alter Table dialog. To remove a column-level unique constraint, you must use the Table Level Unique Constraint dialog.