Adding a Primary-Key or Unique Constraint

When you place a primary-key or unique constraint on a column or set of columns, those columns must contain unique values. The database server checks for existing constraints and indexes:

• If a user-created unique index already exists on that column or set of columns, the constraint shares the index.
• If a user-created index that allows duplicates already exists on that column or set of columns, the database server returns an error.

  In this case, you must drop the existing index before adding the primary-key or unique constraint.

• If a referential constraint already exists on that column or set of columns, the duplicate index is upgraded to unique (if possible) and the index is shared.
• If no referential constraint or user-created index exists on that column or set of columns, the database server creates an internal B-tree index on the specified columns.

When you place a referential constraint on a column or set of columns, and an index already exists on that column or set of columns, the index is shared.

If you own the table or have the Alter privilege on the table, you can create a check, primary-key, or unique constraint on the table and specify yourself as the owner of the constraint. To add a referential constraint, you must have the References privilege on either the referenced columns or the referenced table. When you have the DBA privilege, you can create constraints for other users.