How to enable or disable Constraints?

Example: Enable/disable primary key StudentsPrimaryKey on students table.

ALTER TABLE students ENABLE  CONSTRAINT StudentsPrimaryKey

ALTER TABLE students DISABLE CONSTRAINT StudentsPrimaryKey

How to add primary key to a table?

Example: Add primary key to students table on ID column.

ALTER TABLE students ADD CONSTRAINT "StudentsPrimaryKe" PRIMARY KEY (ID)

Oracle tips?

• When you successfully exit SQL*Plus, execute one of DDL, DCL statements (even if they fail) or use ROLLBACK TO SAVEPOINT X there will occure implicit COMMIT for the open transaction.
• When there is system crash there will occur automatic commit.
• Subqueries are processed from most inner one to the most outer one.
• If we create a synonym for an object in a schema, any user granted on this object can also access this object by using synonym for that object.
• If we modify default value of an existing column on a table, default value will be used for inserts after that point, rows those have been added before with NULL value on the column will not be change.
• Constraints can be added at creation time of tables and also after creation.